From a quick search I think the ATT router seems isolates wifi and LAN to different segmentation so the time machine can not reach the NAS by AFP/SMB.
Some people gives an idea that using another router with BGW may make things easier.
By the way, several topics (also in our community) are reporting some time machine issues after upgrading to macOS 26 include AFP is no longer supported for time machine, and the backup will fail if non-alphabet chars are used as your device name. You may also check on that.