Big issue with my fiber optics internet for Qobuz streaming

I had a similar issue in the past with the way AT&T implements TV over fiber. The solution is to implement VLANs (essentially virtual separate networks). You will need a managed switch to implement. Put the TV and streamer on separate VLANs. The other solution may be to drop AT&T TV and use YouTube TV or similar.

If you happen to use Roon and have ANY wireless activity in your home related to using it- like controlling it from your phone or Ipad or laptop and if as part of your new fiber system the provider has installed various Wi Fi devices- like an Eero router and additional Eero devices that wirelessly extend wi fi in your home then the problem is almost related to the Eero system and its constant steering of signals across two or three bands it employs. So, your phone may be trying to talk to your Roon device via a 2.3g band while the roon device is operating on your 5g band. I've had years of this frustration and eventually went to wired use trying to get all devices on a hard wired network. Still, at times when streaming I'll have freezing / hangups. 

I'm not currently streaming using QUOBUZ, but I can attest to AT&T being generally awful. Luckily, AT&T doesn't offer fiber in my area.

I switched to T-Mobile fiber and couldn't be happier. 2 Gig on ethernet and 900 mbps on WIFI. Far better WIFI coverage in my challenging MCM, aluminum clad house. I do stream Hi Rez files on Amazon through the the provided NOKIA  server/router without issue. I know I'm not utilizing ultimate quality like QOBUZ or Tidal, but the T-Mobile system is fantastic.

This is probably a handshaking issue between Qobuz Connect and your streamer.  I have high speed reliable internet and use a ethernet cat 6a cable from router to high end streamer (Simaudio 891), and I still have this same problem.  Perhaps Qobuz can fix it.  Meanwhile, I have given up on Qobuz for now and gone back to Roon which does not have this problem.  

I have the same ATT modem/router combination that you have.  As has been suggested by others, I used IP Passthrough to take the load off the ATT device for assigning IP addresses, and other router tasks.  My router duties are performed by a Synology router instead.  Having gone this route I have had zero issues with Qobuz streaming in any quality level to my system.  So hopefully you find the same to be true.  

Also, I did not turn off the WiFi broadcasting from the ATT router, but don't have anything connected to it.  I was cautioned by ATT tech to leave some way to get into the router directly for software updates, or for the occassional need to get into the ATT device's settings (which I had to do to set up some port forwarding rules for JRiver and Roon).  I am sure everyone's setup is a bit different but this is how I dealt with it.  

Finally, I make a habit of rebooting the ATT gateway once a week, just to refresh the connection with their servers.  This solved a few other random disconnect issues I had.  I do that via the ATT app and it takes five minutes and of course distrupts network traffic while that is going on, but it's usually done early in the AM or very late at night.