I am having the same problem with the Ring Doorbell 3 "This footage is unavailable" It occured 10:54pm last night for half and hour and 3:26am this morning for 10minutes.
I highlighted the router is 21 feet away which is less than the 30 feet detection limit of the device which is saying poor signal even when next to the router.
I phoned UK support, they say it is 'Ring Daily report' and happens twice a day. Its odd the time and a family member with the same device does not have it
I had this problem to, tried resetting app and nothing worked. Then I restarted my ring doorbell by resetting the power. That fixed the problem for me!