You can't upgrade your firmware because your device can't support it, so you will not be able to update your firmware to the newest.
Firstly, could you provide me troubleshoot file? You can find it in your router WebUI > System > Administration > Troubleshoot, or in your RMS> Management > Devices > Select your device > Action > Export troubleshoot.
To check if your antenna is causing this issue, you will have to verify if your router is providing voltage through the GPS antenna connector. I will send you a PM with a PDF file with instruction on how to do it. If your GPS antenna connector will provide 3.3V +-10% (which is the correct voltage) then most likely your antenna is causing this issue.
One more thing, turn on your GPS and in routers SSH type this command: gpsctl -six and send me an output of it.