@ChrisLaw when you have your SSID hidden, does it restrict your ports and protocols, or put up any firewalls? This could be the reason why, and may need to be something you need to follow up with your ISP about. When you follow up with your ISP, I recommend reviewing this Ring Help Center Article here first in order to establish that when your SSID is hidden, that all of the ports and protocols needed for Ring devices are open.
If this still doesn’t seem to fit the bill as the root cause, I would recommend disconnecting the bridge while on the phone with your ISP, try reconnecting to the hidden SSID, and see if they can see the reason why it’s been restricted on their end. Normally when any devices (Ring or not) are attempting to connect to your router and are successful or fail, your ISP should have logs of this in detail that may isolate the cause of this. Let me know how it goes! 