So I have seen a lot of posts WRT this "weak GPS signal" error using Apple products but I am having this exact same issue with Android:

As of 07/06/2020

DJI Mavic 2 Zoom, latest DJI GO 4 app and all updated firmware.
Samsung Galaxy S8 with latest OS/firmware.
Installed GPS status app on the phone just to check satellite connection and GPS status on the phone.
Ensured location is enabled on the phone and set to high accuracy (uses Bluetooth and WiFi a well as GPS signal for higher accuracy)

Standard RTH (from launch location) works perfectly every time but RTH to controller throws the ""weak GPS signal" error every time. Tried the unplugging micro USB lead at the controller workaround which does not work for me.

To be clear, the S8 has solid satellite connection with 12 or more satellites locked and lat/long of the phone acquired, despite this the controller disputes this fact with the "weak GPS signal" error so this has to be an error in the DJI app or firmware. Also tried using another device (Huwaei P Smart) with the same results - weak GPS signal so this isn't a phone issue. Not a cable issue either as I am using the stock cables that come with the Mavic (one USB C type on the Galaxy S8 and one micro USB B type on the Huawei PSmart).

Really frustrated that I can't get the Mavic to return to the controller location rather than the launch home point, mainly because I am a sailor and want to fly from the boat to get sailing footage, RTH to launch point would be disastrous on a moving sailboat.

Anyone else having this issue on Android devices with DJI GO 4? Any working workarounds/fixes?

When is DJI going to deliver an update that allows this critical piece of functionality to work?

Setting your controller as the home point requires a very strong GPS connection to your phone. GPS connections change slightly when you move and I don't suggest you use this feature trying to land your aircraft on a moving boat since GPS signals mostly aren't strong. Also, ensure that you set the new home point at least 50m away from your take-off spot.

DJI Co-Pilot Posted at 7-7 02:56
Setting your controller as the home point requires a very strong GPS connection to your phone. GPS connections change slightly when you move and I don't suggest you use this feature trying to land your aircraft on a moving boat since GPS signals mostly aren't strong. Also, ensure that you set the new home point at least 50m away from your take-off spot.

Setting your controller as the home point requires a very strong GPS connection to your phone.
There's no such thing as a strong or weak GPS connection.
You just have GPS or you don't.

GPS connections change slightly when you move and I don't suggest you use this feature trying to land your aircraft on a moving boat since GPS signals mostly aren't strong. This doesn't make any sense.
But it's never a good idea to land a drone on a boat.
Hand catching is the only way to go.

Also, ensure that you set the new home point at least 50m away from your take-off spot.
It doesn't matter what distance away the homepoint is reset to.

Hi. I am sorry for the trouble this has caused. With regards to this concern. Kindly please refresh the firmware of the DJI Mavic 2 Zoom using the DJI Assistant 2, after the firmware refresh please calibrate the compass and the IMU of the said drone to isolate the issue. There might be a strong interference in the area that is causing this issue, please try flying the drone in a different location and please keep us posted for further assistance. Again, I am sorry for the trouble. Thank you.

Hi,
I was wondering if you finally managed to solve this issue by refreshirng the firmware as @DJI Paladin said. I am having the same problem with my Mavic Air I. No way to set the RTH to the controller. Always the same message... "GPS of the mobile device is too weak".
Any help of how to make this work would be appreciated.

Ellen73 Posted at 9-5 08:11
Hi,
I was wondering if you finally managed to solve this issue by refreshirng the firmware as @DJI Paladin said. I am having the same problem with my Mavic Air I. No way to set the RTH to the controller. Always the same message... "GPS of the mobile device is too weak".
Any help of how to make this work would be appreciated.

Hi,
No this issue was not resolved with the standard controller. I did upgrade to the smart controller and that solved the issue for me.

Ellen73 Posted at 9-5 08:11
Hi,
I was wondering if you finally managed to solve this issue by refreshirng the firmware as @DJI Paladin said. I am having the same problem with my Mavic Air I. No way to set the RTH to the controller. Always the same message... "GPS of the mobile device is too weak".
Any help of how to make this work would be appreciated.

Paladin's "advice" would have been 100% useless as the issue had nothing to do with the firmware, the drone's compass or IMU.
The issue was all about the app not recognising the GPS data from the tablet, due to some unknown configuration or programming issue.

Correct me if I am wrong but the number of satellite locks of the phone relates only to the phone's  ability to locate itself and therefore the controller. If I am correct then this is of significance ONLY if,
a) you wish to reset the position of the homepoint to that of the controller e.g. if you were moving and wanted the drone to return to you and not the original home point.

Or perhaps
b) to show the position of the drone realtive to the controller whilst the controller is moving or turning.

The number of satellite locks of the phone HAS NO BEARING on 'strength' of the GPS signal reported by the App. The App is reporting the number of satelllite connected to the DRONE and NOT the phone. The drone has its own GPS reciever which is independant of that of the phone. Case in point, I often fly my drones with the phones "location" services all switched  off so the phone has no idea of where it is. The drones, however, still know where they are.

Sean-bumble-bee Posted at 9-6 06:07
Correct me if I am wrong but the number of satellite locks of the phone relates only to the ability to locate the phone and therefore the controller. If I am correct then this is of significance ONLY if,
a) you wish to reset the position of the homepoint to that of the controller e.g. if you were moving and wanted the drone to return to you and not the original home point.

The number of satellite locks of the phone HAS NO BEARING on 'strength' of the GPS signal reported by the App. The App is reporting the number of satellite connected to the DRONE and NOT the phone.
DJI contributes to the confusion by the message saying Weak GPS Signal, when the issue is No GPS Signal (that the app can detect) even though the phone has perfect GPS reception and location data.
It's been an issue with Apple devices for a few years now.