Make sure you have inserted a Nano SIM card that comes with Talk Time and Data. Caller ID is also needed for some countries. Remember to top up the credit and activate the SIM card too.
While inserting the SIM card, make sure the SIM card tray is inserted in the correct direction. The arrow of the SIM card tray should be pointing upwards.
Sometimes, when you need to remove the SIM card from the watch, do remember to restart the watch after you have reinserted the SIM card again.