My Apple Watch won't send iMessage on cellular, it always tries (and fails) with SMS

[fezzzer]

As above. This has persisted from my Ultra 2 to the Ultra 3. I have checked and both are logged into the same Apple Account. Has anybody experienced this?

Edit: I've tried the steps here.

 

[poocat]

I am having this issue with both my wife's watch and mine. Have tried all steps above, but not the below, which was suggested by ai.

On your iPhone, go to Settings → Apps → Messages, turn off iMessage

1. Go to Settings → General → Transfer or Reset → Reset Network Settings (this wipes Wi-Fi passwords but forces a clean iMessage re-registration)
2. Restart the iPhone
3. Turn iMessage back on in Settings → Messages
4. Wait for it to fully activate (may take a few minutes)
5. Check that Text Message Forwarding appears and your Apple Watch is listed
6. On the watch, verify under Settings → Messages that iMessage shows as active

 

[poocat]

Also this bug is entirely reproducible, any transition from wifi/phone to cell only fails to enable iMessage (text box in Messages switches from iMessage to SMS as the connection updates, and never moves to iMessage again).