I had a similar bad ONT (maybe it was the router, I don't recall now) right after installation. Not ideal, but not unexpected.
About a year or so in, I had a hard fail. On the line with tech support for over an hour. Try this, try that, reboot this, reboot that - the whole script. ATT decides I need yet another new router. Set that up for delivery in three days. Just after setting that up, and hanging up the phone, I notice ATT truck over at my neighbor's house. "What's up?" I ask. Evidently the whole neighborhood was out. Very frustrating that I wasted over an hour on the phone with ATT Tech support, when the base problem was systemic to the neighborhood. Had to drive to UPS when the (new) shipped router arrived to return it to ATT - unopened. Left hand doesn't know what the right hand is doing.
More problematic is the firmware updates. My Fiber line from ATT has died 3 times over the last 24 months because of borked firmware updates for the PACE router. ATT can't handle quality control at all on their firmware. It's not their strength. The current Pace router firmware is broken, and workarounds must be used. There's other threads discussing those problems.