OP :
1500 is best case, but it may be constrained by upstream issues. Leave your router alone if everything else is working, but check if your streamer has a setting for it. Make sure it's equal to or less than whatever your router is using, and as an experiment, try 1,300 on the streamer, if possible, and if not, try lowering the max MTU on your router to 1,300.
Technically, 1,300 MTU is not "as good" as 1,500, but practically, the performance delta is negligible. The issues arise when you try to use a larger MTU than your network actually supports.
I did by the way find similar issues to yours:
https://www.facebook.com/groups/streamingmusicmatters/posts/3840512299574313/
I still think you should try my recommendations. Try the DNS route first, then try if possible to reduce MTU to 1300. AT&T should support 1500, but it's always possible there's 1 network branch out there that is the offramp to Qobuz that doesn't.