Black video, Weak connection - all healthy tests

Hi all, I've been having a frustrating couple of weeks with the Ring Pro. I've been setting up a connection to a new dedicated WiFi router (Asus RT-AC59U) instead of the Virgin Media SuperHub 2 as I was having intermittent WiFi issues. I have a connection to the doorbell, and the phone and chime ring when the doorbell is pressed, so there is full connectivity. The problems start when trying to look at live view and recordings. Live View says the phone connection is weak (full WiFi, same network, tested on both 2.4GHz and 5GHz for both phone and Ring) whether I'm on WiFi or 4G. When logging into the Ring website, I have video recordings logged but they are all black. I'm also not getting any motion notifications. I've been through all the new mode settings to make sure I have all the notifications active. It's been working fine for 6 months, and I changed the adapter from a cheap Amazon one to an official Ring Power Adapter yesterday. This has made no difference. I've ruled everything else out - distance to the router is 6 feet and it has a strong connection, phone connection is strong, power is good etc. All the health checks come back fine - strong WiFi, good power, speed tests show 230Mbit down and 21Mbit up, all other devices are working better than ever on the new router on both 2.4GHz and 5GHz, the whole network has never worked better - except for the Ring Pro which hasn't worked for 2-3 weeks on the old or new networks. There are recordings on the account, including the snapshot photos, which are all black or not viewable. Any help would be appreciated as I'm currently paying for a service that's not functional. Kind regards, Chris