Since sleep is calculated by web Connect based on data from the watch itself, I doubt that the problem is the phone. What happens if you manually enter your sleep start and end time?
If you get a graph and light/deep/rem stats then I think the issue is Garmin isn't able to figure out a start or end time. In my case, it almost always has a good start time but gets the end time wrong. I adjust it manually virtually every day.
