Hello,
Did you check, if, after SSID disappearance, you are able to login to the device via SSH?
If you can, I would like you to generate and attach a troubleshoot file to your question. Once the issue occurs, login to the device via SSH and execute the command below (do not mind possible errors):
Download WinSCP. Login to the device using the downloaded application as instructed here. Navigate to /tmp directory. Find and copy troubleshoot file to your computer and attach the file by editing your original question.
You could also try firmware update vis CLI/SSH as well. Instructions are provided here.
Otherwise, could you try firmware update via bootloader menu, as instructed here?
Also, could you provide the details of your device configuration, that you have done done over default settings?
Best regards,