You worked in IT a long time, you should understand hardware failures/problems a little better.
( I understand that he missed the bad ONT on a first install)
If the initial tech had you up and running 100%. When he left, how was he supposed to know it would go out after you fumbled with it. ? Wonder if psychic techs get paid more?
Second call, they rebooted the ONT and sent a tech out who didn't even look at the ONT, basically just looked at it and said it's working nothing I can do.
Would you have felt better if he "looked at it" with his eyes THEN said "nothing he can do" ?
It was working fine when he was there. As you can see from tech support, they seemed to insist reboots would resolve it. The field techs know the same thing. Most of the time, once its up, it stays up. Some techs may not be comfortable replacing (what is probably) good equipment. Especially when it could be the router, the wiring, or the ONT. Where was YOUR psychic abilities of powering down the ONT to replicate the connectivity issue, and PROVE it to the first repair tech. ?
The problem here was that you had an odd problem that rarely happens. If your internet had been out completely, Im sure you would have been repaired on the first tech visit.
Well, at least youre good now.