On the watch, swipe across to the QR Code app and tap the RegCode for the registration code.
You must be using an activated SIM card, inserted correctly, and with the watch rebooted after installing the SIM.
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 check your watch for updates)
Then 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.
If you're using the Angel Watch SIM selected at the moment of purchase please confirm it's active. (Steps here)
If using an international SIM card, make sure that you select your network provider's Android APN setting or contact them for assistance.