Facebook and Local DNS caching
I just changed internet providers (away from Comcast to local GloFiber), and a problem popped up. When accessing Facebook from iOS 14 (iPhone and iPad), it would initially work, but after a short time, I would get "unable to establish a secure connection" error. If I switched to cellular, access works, If I then turned wifi back on, it again worked for a short amount to time.
I realized it might be DNS related, and turned off "Local DNS Caching", and the problem is now gone.
Eero 6 Pro, v6.4.0-2092