I have been using an external HRM with the series 2 and 3 without any major issues. Since updating to Watch OS 5 and upgrading to the Series 4 from a Series 3, I haven't had much success. I have since updated to Watch OS 5.0.1 and hasn't improved the situation.
It looks as though a connection to the external HRM isn't maintained when looking at the Bluetooth setting on the watch?
It looks as though a connection to the external HRM isn't maintained when looking at the Bluetooth setting on the watch?