This is all because AT&T believes that the edge of their network is not the PON ONT/OLT, but rather, the router they issue you. If you want to be on their network, you have to use their router as some part of the chain.
My latest discovery is that in doing this, the router can actually get super hot operating at gigabit speeds for extended periods of time. When this happens, it magically starts dropping packets. Solution? Aim a fan at the router so it has "thermal management."
Total. Garbage. I'd switch to Spectrum if they had decent upload speed, but alas, they don't in my building.
[0]: https://particle17.xyz/posts/amplifi-alien-thoughts/#appendi...
[1]: https://www.att.com/support/article/u-verse-high-speed-inter...