Having same issue Centurylink modem/router Technicolor C2100T. I also have multiple ring devises and have now spent 4hrs on and off with multiple people on help line. No solution here but the Ring customer service said they were eselating this to resolve.
All of the work you said you did and troubleshooting I also did to no luck. I also had it “connect” the first time… but according to the customer service this was some kind of false posative. I didn’t try and connect the lights, I actually connected it to my guest wifi on accident and used the change network button… which then resulted in never being able to connect again. I can see it connect to the network as a client for a couple seconds but then drop off immediatly.
On the device I notice it goes from (slow blue light blinks -> fast blue light blinks -> Solid blue light blinks -> phone reapears as a client -> Ring bridge appears as a client -> Ring Bridge drops to 0% -> Blue light returns to fast blinking -> Ring bridge drops from client list -> Blue light blinks slow again -> Phone pops up and asks to connect to the ring devise again. -> says fails to connect to ring device -> pops up with sometimes devise doesn’t connect screen with try again button. )