As a process of elimination can you insert your SIM card in another phone to see if the SIM card is working?
If.it isn't then you'll need to contact your Mobile Network Customer Services.
If it works then can you try another SIM card in your phone.
Did your Network supply the Samsung Galaxy s8 phone to you ?
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
I am having a similar problem, I've been using my two sims for basically everything until this evening, my Sim1 card tray no longer recognises any sim card, it's really frustrating. It's a galaxy S8 in case anyone can profer a solution.