I have done all the updates and checked what could be the cause but I can't get the RC Plus to link with the M300 we have.
Anyone have any ideas on what to try next?

Is it because it has an original RC paired already?

Hi, Craig_Tas. Thank you for reaching out.  We recommend to re-update the firmware. Please download the offline firmware package of the corresponding version from DJI's official website and upgrade or downgrade the firmware according to the offline update method using DJI Assistant 2. Official download link: (https://www.dji.com/downloads/products/matrice-300).

Please follow the steps below for the offline update:
1. Log into the official website. Download the offline update firmware package of the corresponding device.
2. Save the downloaded offline update firmware to the microSD card or the root directory of the USB drive.
3. Insert the microSD card into the remote controller microSD card slot. If a USB drive is being used, insert it into the USB port on the top of the remote controller.
4. Mount the gimbal onto the aircraft. Then, link the aircraft with the remote controller. Go to UAV Health Management System. Select [Firmware Update] - [Offline Update].
5. The app will automatically identify the firmware package in the microSD card or the USB drive. The user selects the device to be updated. Click [Update All]. Follow the app prompts to finish the offline update operation.

DJI Gamora Posted at 5-31 19:10
Hi, Craig_Tas. Thank you for reaching out.  We recommend to re-update the firmware. Please download the offline firmware package of the corresponding version from DJI's official website and upgrade or downgrade the firmware according to the offline update method using DJI Assistant 2. Official download link: (https://www.dji.com/downloads/products/matrice-300).

Please follow the steps below for the offline update:

Both the RC Plus and the M300 are on the latest firmware and I have refreshed them both via the DJI Assistant 2 Enterprise. I initiate the linking procedure and it counts down to about 30 seconds and says linking is complete but it doesn't complete because if you do the same with the original controller it links in seconds of holding down the power button on the craft for 5 seconds.

When I am back at work next week I will attempt the above and update you. Thanks again for the fast response DJI Gamora.

Craig_Tas Posted at 5-31 19:42
Both the RC Plus and the M300 are on the latest firmware and I have refreshed them both via the DJI Assistant 2 Enterprise. I initiate the linking procedure and it counts down to about 30 seconds and says linking is complete but it doesn't complete because if you do the same with the original controller it links in seconds of holding down the power button on the craft for 5 seconds.

When I am back at work next week I will attempt the above and update you. Thanks again for the fast response DJI Gamora.

The way I understand it, there's two seperate M300 firmwares depending on which RC you plan to use. There is no firmware that works with both the RC Plus and the Smart Controller. So if you're saying the original controller links in seconds, that would indicate that you're on the wrong firmware in respect to the RC Plus version.

Johnnokomis Posted at 5-31 20:43
The way I understand it, there's two seperate M300 firmwares depending on which RC you plan to use. There is no firmware that works with both the RC Plus and the Smart Controller. So if you're saying the original controller links in seconds, that would indicate that you're on the wrong firmware in respect to the RC Plus version.

Well that makes sense in that its another firmware but not being obvious or even being able to be done the normal way. Thanks for the information I am sure now it will work correctly.

Craig_Tas Posted at 5-31 21:20
Well that makes sense in that its another firmware but not being obvious or even being able to be done the normal way. Thanks for the information I am sure now it will work correctly.

From : https://dl.djicdn.com/downloads/matrice-300/20230518RN/Compatible_Firmware_Update_Guide_for_M300_RTK_and_M350_RTK_EN.pdf

Craig_Tas Posted at 5-31 19:42
Both the RC Plus and the M300 are on the latest firmware and I have refreshed them both via the DJI Assistant 2 Enterprise. I initiate the linking procedure and it counts down to about 30 seconds and says linking is complete but it doesn't complete because if you do the same with the original controller it links in seconds of holding down the power button on the craft for 5 seconds.

When I am back at work next week I will attempt the above and update you. Thanks again for the fast response DJI Gamora.

Hi, Craig_Tas. Thank you for the reply. Please keep us posted.

That has resolved the issue, thanks again for all the assistance.

https://youtu.be/Opx4L8uAI5g check this video even is not în english language you will understand quite easily

Craig_Tas Posted at 6-6 18:01
That has resolved the issue, thanks again for all the assistance.

Hi, Craig_Tas. Great to know that the issue has been resolved. Should you need further assistance, don't hesitate to reach us here at DJI Forum again. Have a nice day.