Originally Posted by plunet

First thing you need to do is to work.out of your S21 phone is CDMA (USA) only or supports GSM/LTE (rest of the world)

"You can check if your phone is GSM or CDMA by going to Settings, then About phone, then Status. If your phone has an MEID or ESN number, it's CDMA. If it has an IMEI number, it's GSM. If your phone has both, it supports both CDMA and GSM."

If it's CDMA only then you need a different phone to use it outside of the USA.