Ring Doorbell 3 cannot connect to my Tmobile 5G hotspot.

Ah, this is no good. I haven't seen any reported issues with setting up this Ring device and I'd image the set up was done properly if this other devices are connected. Have you contacted Ring support to see if they have a solution? Other folks from their support team may have some ideas. We can have our Tech Care team file Trouble Ticket to look into your network connect as well.