@Rice Sounds like a fluke. If it really is the case that your NAT doesn't do hairpin, the only explanation that makes sense is Wi-Fi Assist.
I bet you wouldn't be able to replicate the issue if you turn on Airplane Mode...
That can be turned off in Settings to test.
Settings > Cellular and scroll down past the list of apps
video link here of same issue in airplane mode. Note the airplane at the top. My router is a AT&T fiber BGW320 with a unifi u6 long range access point. It doesnāt support hairpin/loop back NAT.
WiFi assist is and always has been OFF on my iPhone.
That worked.
Okay, so that rules that one out.
I think this might explain the situation:
No, the AT&T Gateways do not fully support NAT loopback (or hairpin as it is sometimes called). Sometimes it will appear to function, albeit very slowly.
This is really the worst situation possible for us: It acts like it works the first time we try it, but then fails right after it. I'm not really sure what it is that we could do with this kind of defective router.
"or acquire and use another router with the Gateway in IP Passthrough mode."
Put the AT&T gateway in IP Passthrough (bridge) mode
Unfortunately this is not feasible. The way att implements ip pass through it causes a double NAT which is not acceptable for my use. Att further complicates it by only allowing fiber authentication with their gateway routers. How I miss my old edge router.
The app worked fine with this router for over 2 years. I guess I need to request a different router and hope they have switched to a better gateway.
With no other alternative routers available from AT&T, I decided to spin up a pi-hole and block my ://ff9xxxxxxxx.u.channelsdvr.net hostname. This has fixed the problem.
It looks like it is possible though... basically one device connected to the router gets the public address. So setup a pfsense box or something similar and connect it to your router...