Silly question, did you point the phone's camera at the watch's screen when prompted during the pairing process? Once you do that the watch's screen should update with a design similar to the back of the watch.
Example here, directly linked to the appropriate time in the video:
It sounds like it may be stuck in a pairing loop or something like that. I've seen a few references to that while searching online but there didn't seem to be any concrete solutions. Have you tried the usual troubleshooting steps of turning the phone and watch off and on again?