Hi Davie,
I’d suggest you try working through our video troubleshooting guide to see if any of those steps help you diagnose the problem.
Note that while the companion power supply point suggests the companion computer may require a 5V 2A supply that’s for when the supply isn’t powering anything else (e.g. pixhawk, servo, etc). We generally recommend a 5V 6A supply like this one. Depending on when you bought your ROV, you may have our previous 5V 3A supply in which case it’s suggested that you upgrade to a 5V 6A one if you’re having issues with undervoltage throttling the RPi performance (if it’s an issue, you should see a warning for this in the companion web interface’s System page).