There's no actual Bridge mode, PassThrough is 99% close.
Assuming you setup Passthrough mode correctly on the AT&T BGW modem, and since you've (or the tech did) changing the LAN to 192.168.2.1 / mask 255.255.255.0
What you need to do on the Router: Set 192.168.2.1 for you, 192.168.1.1 is what I have because the Modem is set to 192.168.10.#/255.255.255.0 subnet (192.168.10.254 is my BGW modem)
View attachment 63225
And your DHCP settings to assign ASUS LAN IP's in the 192.168.2.# subnet, starting and ending address. LAN devices will get have 192.168.2.# (within the starting and ending range). I use 192.168.1.20-254 for my LAN IP's, remember in my case I chaged the Modem's LAN IP from 192.168.1.# to 192.168.10.# withem the Moden the haveing 192.168.110.254 as a result. This will get your LAN devices with 192.168.2.# addresses
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Change the 192.168.1.1 to 192.168.2.1, reboot router, done
Again, in my case, changed the AT&T modems's LAN and kept 192.168.1.1 on the ASUS LAN side, my modem is 192.168.10.254 so I can get to the modem from the LAN. This will be the case for you, the modem management will be 192.168.1.254 from ASUS LAN devices with 192.168.2.# IP Addresses
For the AT&T modem MAC address used to enable PassThrough is the MAC Address of the WAN ethernet port on your router, and make sure it's set to DHCPS-fixed so that the WAN IP the router recieves from AT&T is a Public IP and stays fixed.
View attachment 63226
Assuming all correct, when you reboot the router the Network Map will show the public AT&T provided IP for the WAN (your routers) address, in my case 99.150.x.x and your authentication issue with your streaming providers will disappear. I'm suggesting you take a second look because the issues you're describing with the streaming providers is a common one if double NATted (PassThrough not configured correctly).
View attachment 63230
Good Luck, YMMV!
I only have my AX88u on the AT&T modem, WiFi radio's disabled on the modem as ASUS does a much better job, the Modem's firewall disabled too, again the router does a much better job, last thing. I have QoS on the router turned off/disabled, then again I have 1Gb and with it enabled it cut my over all bandwidth to 400Mb-500Mb and disabled ~940Mb in both directions.