It's just so bizarre.Did the same for me 3 days ago on my iPhone 7 plus AT&T
Yesterday tried 6s plus att and was able to get to follow- but only for 5 minutes or so- then it disconnected
I was able to set my position as a new home point on the 6S plus but not on the 7+
Did this five different times.
Tried 7+ AT&T many many many times
No joy
Very very disconcerting
I wonder if it's the iOS.
My wife's work phone which is AT&T works just fine with all my birds but it iOS 9.85
UGH!