After successful activation SIM activation, remove and re-insert the SIM card.
Then on the watch go to SETTINGS-REBOOT.
When the watch restarts, swipe down on the watch screen and confirm you should see 4G with at least two bars of service. (If not, please go here)
If you see 4G with bars on the watch, go to the QR Code app and tap Reg Code to register with the app. If you see 0's under the QR code, you may need to wait 5-10 mins for the code to change. If it doesn't, please reboot after 10 mins and it should change by itself in less than five minutes.
When the code changes to alphanumeric by itself, you'll be able to register the watch.