FaceTime Poor Connection over WiFi - no issue over LTE

I've been having some FaceTime issues over the WiFi for some couple of years now. I use Apple routers throughout my house and have this issue over iPhone, iPad, Mac. In total I have around 9-10 Apple devices at home and this is an issue I face with every device. Basically when we initiate a FaceTime call, it starts as it should but then few minutes into the call we get a black screen saying, "Poor Connection. Video will resume automatically when the connection improves". I have AT&T Gigabit connection so bandwidth is not an issue. This is for all FaceTime calls not just one individual in one particular area. Even WhatApp video calls go through the same routine. The moment we turn off WiFi and turn on LTE, everything returns to normal i.e. we never have this issue over the LTE.

This week I bought a Nest Video Door Bell and each time someone rings the bell, we get the prompt on our phones right away but it takes 40 plus seconds for the video to start but when we turn off WiFi, the video starts in 4-5 seconds (over the LTE).

Any idea what is going on? I have checked my router settings and nothing seems to be out of the ordinary. My buddy has same routers with same settings and he has no issue. FWIW, no issues ever with Netflix, Amazon, Apple TV, or any other streaming device.