DNS resolution issues

Hello,

We have been running a school network using a Synology 2419+ as our Active Directory and DNS server. It was all working beautifully until we switch internet providers from Spectrum broadband to dedicated AT&T fiber. Initially AT&T worked fine; within two weeks we started having intermittent internet issues which we narrowed down to being a DNS issue. We had the DNS resolution turned on and forwarding set to Google (8.8.8.8) and Cloudflare (1.1.1.1).

Through testing we found that our setup continued to work on a Spectrum line but every time we enabled the resolution service on AT&T, within a couple weeks it would fail again. Looking at the DNS log when using AT&T we would get denials from 8.8.8.8 and 1.1.1.1 for DNS resolution.

We have had multiple tickets with AT&T, had techs out a couple times, and each time they say "The issue is on your end." Anyone had this kind of issue before?