Presume you are looking to have your 4G version watch 3 work with your phone and also stand alone on an O2 plan I believe O2 are one of the carriers who support this ( think is only O2, Vodafone and EE ) where you add your watch on to your monthly phone plan ( incurs an extra cost on your plan ) I don't know any reason this would not work with your s21 but suggest you contact O2 and confirm with them - am sure they would also advise on setting up etc and the extra monthly cost ( I have my watch 3 4g version added on my Vodafone phone plan ( extra £7 per month ) for my Note 10+ and it works really well )
