Tricky and funny SOLUTION (probably works for someone else too): I updated my S21 yesterday and then no signal/not registered/emergency calls only. Tried all tricks restarting phone, taking sim in out, resetting network, APN reset etc but nothing worked. My wife has A52 and same network so I swapped the sims with her phone. I found that her sim was working good on my phone but my sim was not working in her phone which made me think that the update process must did something to the sim and I may have to replace my sim. After multiple attempts of 'registering on network', my sim started working on wife's phone too so now both sims working. I swapped the sims back and now both sims are working in their phones as they used to. It's strange if the update process affects the sim in some way, or it could be just a technical glitch.
Samsung should provide a solution for this since it's not easy to change your phone once you invest in one phone with all your data.
And at one point I thought that even if I replace my phone, that might be updated too and might have the same issue then where it gonna end, just keep searching and trying different phones around to find the one which is not updated and works on my sim?
