I’m on a similar set up to you except I use a Connect and a Connect Quattro (in the UK).
I’m kind of hoping it’s related to the beta builds otherwise I’m at a complete loss as to what the problem could be…
ATV on tvOS 11.2.6 runs Channels app flawlessly regardless of whether wired to unmanaged gigabit switch or on wifi. This confirms network and both connects have no issues.
ATV on tvOS 11.3b4 - Channels is pretty much unusable regardless of whether wired to switch (same wires and port as above) or on WiFi (same location as above). Could it be that the beta builds have affected the way Channels can handle the video and/or audio formats in the UK?
Not sure if this helps but same issue with InstaTV. I tried that first on the ATV on the beta builds and as that too has problems it led me to think there was an issue with the Connects but the fact they work perfectly on the other ATV (in both Channels and InstaTV) and all other devices has ruled that out.
Are there any logs I can produc/view in Channels that might help. HDHomeRun logs don’t show anything of note.