Also having the exact same issue, have spent numerous hours on the phone to 3 which was little to no help, then spent an hour on the phone with apple and we reset watch and phone and set them up from the start, all goes great until we get to the set up cellular part where it takes you to the three site to log in, enter email and password then it says error one of the fields is missing. Still no further forward with this and the service from three has been appalling.
Did you manage tofind a resolution to this?
