No connection to Trident, no booting after firmware update

just got a used Trident and have problems connecting it. I can charge the batteries (Headlights turn on when I connect the charger), I can connect the surface unit, but then only the first two LEDs power up (Power and Wifi, the OK LED stays off). The app won’t find the Trident. The previous owner said he updated the firmware and then lost connection. When I connect my MAC to the Trident’s Wifi, arp -a (in terminal) won’t get any results.
Is there a way to get more info about what’s going on? Can I put the firmware directly on the SD card?
Any other advise?

Now I opened the ROV and checked that the electroics is working, LEDs of the Raspberry are blinking. Therefore I think that I just need to put a new firmware image on the Raspberries SD card. Is there any way to get it as a download?