For as long as I have owned the Inpire 2 - which is 2 years -- I have been unable to access firmware updates with my house internet.I have very fast internet, I have a great router, wifi hotspot, etc. I've never had trouble with any other site, and I can access all DJI sites fine.

But, if I connect my inspire 2 to the DJI Assistant 2 app, and check for firmware updates, it fails and says check the network.... well, the network is fine.

If I tether my cell phone with the laptop, it then works.. I'd really like to figure this out since it's a major pain in the rear!!!

This same issue happens with other DJI products as well, so it's something UNIQUE to DJI. Maybe cause it's all i china??? The great firewall?
I don't have a firewall here, so not sure what might be causing it. Anyone have a clue?? Or things to test??

I know it's been quite some time since your post but I'm having this same exact issue with my new DJI Mini 3 Pro and it's RC controller. I can access the internet over my home WiFi with the controller but I cannot download any firmware for either the drone or the controller. When I try to check for firmware updates with the RC controller over my home WiFi I get the following error message: Server error. Wait a moment and try again. (Error code: -8). Interestingly, just like you, when I link my cellphone and use it as a hotspot to the RC controller I can download the firmware update. I can also do it over DJI Assistant 2 but again I have to tether my cellphone to my laptop and use it's 4G LTE hotspot connection. Have you had any fixes for this? DJI hasn't been able to help me yet. Thanks.