Apple Watch set up

Final update and resolution!: The watch need to be moved from Sprint to T-Mobile. You will need to get with a tech person at TMobile (I did again via chat) and they took the EMEID of the watch and were able to migrate it. It took an hour or so to propagate but when I go to Activate Cellular via Watch app it finally asked for my login credentials and then on to assign a line to the watch. I have, as of now, completed this activation.

Takeaway: If you were with Sprint and recently migrated to T-Mobile due to their acquisition, you may find that the additional devices paired with your mobile may need to be manually moved to T-Mobile as well.

<whew!>

I spoke with Apple support last evening after online support escalated. We are pretty certain this is isolated to T-Mobile users. If you are experiencing this same issue and are on T-Mobile, please speak up.

It appears there is an interstitial page the cellular activation page is trying to reach, but fails due to the Hazelcast server error.

My final test last evening was to check my son's devices. He has a 6s with a watch 3 already running cellular. When I go to the same screens to update cellular, it fails throwing the same error. Thankfully his setup was activated a few weeks ago.

Apple engineering has been notified but I think we will find this is a T-mobile issue and ideally someone will realize the bottleneck in cellular activations and correct this sooner than later.

Again, if you are experiencing the same, please speak up and let's see if it really is T-mobile devices choking on this.

I'm having this same issue and have been working with online and in store techs with zero solution. I'm a Sprint user, recently moved to T-Mobile SIM. The watch pairs fine with the phone but when I go to Set Up Cellular either in the initial setup of the watch, or after, the T-Mobile page open in Watch app but errors out with 'Hazelcast instance is not active!' No one (multiple online sessions and trips to the service provider) have ever seen this nor any attempts to troubleshoot have resolved going on 3 days now.