Good to confirm 
Ok, so most likely the hardware connection from the onboard Raspberry Pi to the topside is working (although if you bought your Fathom-X boards between May and June this year you should do a quick check for this issue).
From there it’s a question of topside network configuration, and the software running on the onboard computer. If the network configuration is correct then assuming the onboard computer is working properly and running expected software you should be able to access the web interface at http://blueos.local or http://192.168.2.2 if you’re running BlueOS, or http://192.168.2.2:2770 if you’re running the old Companion Software (the web interface may take a minute or two to become available after turning on the vehicle). If that doesn’t work then your SD card may be corrupted, in which case you can try flashing on replacement software (BlueOS or the old Companion Software).
Once you have access to a web interface you should be able to see if there is a vehicle registering as connected, and a camera for the video stream. If those are present then try QGC and see if it shows the video stream and the vehicle telemetry.