Have you transferred your previous phone data to your Samsung s21 if that's the case maybe something that it sent over dose not work or is glitchy so maybe thats the reason why it is not working who knows.
I've had the exact same issue, multiple random restarts up to 15 times a day, I've sent my S21+5G to the iCure repair centre in Granville (why does Samsung use dinky 3rd party repairers, they're a multi-billion dollar global corporation!) and the iCure centre found no fault and returned the device. The issue has become worse, I can barely use my phone. I'm actually on call with the Courts and need a working phone, yet each time I am told to send it back to the Service Centre (which means I am without a device for up to a week or two). Samsung Members tell me the error logs indicate hardware issues with AP and DRAM, yet the service centre run a few tests and say all is fine. Completely unacceptable and I'm appalled Samsung can treat customers this way. I suggest anyone with this issue think about going to Fair Trading and also look at your ACL rights.
