Why is my SIM card not working?

Hello All, I purchased my Samsung Galaxy S21 (unlocked) in December of 2021. I was overseas on vacation and removed my SIM card to not get random charges. When I returned my SIM card did not work. I got a new SIM card, no dice. Finally, I bought a new phone and put the SIM card in, it works! How can I start trying to repair the problem with my Samsung Galaxy S21 SIM card port or is there a software solution?