sim card

Is the SIM card the one from your old device?  If it is a new SIM.  Has the SIM card been activated by Customer Care or in a store?  

T-mobile is the carrier service for qlink wireless. Qlink wireless is who I'm under. I've used the simple card in two other phones w/out any issues

My Galaxy S21 would tell me there was "No SIM" a couple times every day. Restarting the phone was the only way to get it to "find" the SIM card. I have the "unlocked" version of the phone. I tried all the suggested fixes, and nothing worked. I tried a new SIM card, and that did not help either. Finally, I asked T-Mobile to switch me to the eSIM. That fixed the problem.

Could it be a problem with the SIM card holder in the phone?  Yes it could.  I do not think it is a physical problem, however, because when I restart the phone, the SIM is recognized just fine.  It also tended to lose connection to T-Mobile when I would drive to a new location.  My guess is that the problem is either some kind of strange glitch in the phone with the SIM card reading circuitry or that T-Mobile is not connecting properly to the phone since it is unlocked.  I know an unlocked phone should work fine on any compatible network, but I saw a comment where T-Mobile told someone that an unlocked phone may not be compatable on T-Mobile’s network.  I know my phone is unlocked because it does not start up with the T-Mobile logo.  It is a Samsung Galaxy S21 SM-G991U1.