So last night I posted for help with my Ubiquiti Dream Machine Pro and ARC set up. Was getting the multiple NAT error.
It was suggested that I need to get into the ISP modem and set to bridge mode. Great! But, AT&T does not allow bridge mode on its modems.
After a quick search, I found this information for setting up IP passthrough.
Enter the modem set up via IP address > Firewall > IP Passthrough > Set to “Default Server” > Choose from devise list or manually enter your 3rd party router’s IP address > Save.
Opened Roon, went to settings, to ARC and it immediately configured the Core. Then the iPhone and iPad.
Hopefully this helps others.