IMS stands for the IP Multimedia Subsystem, and it is part of an Internet Protocol that also comes pre-installed on all Android devices.
If your IMS registration status is “not registered”, it means that your VoLTE or Voice over LTE is not active, and you can’t make or receive calls and can’t send text messages as well.
It's not a software issue that fixes on next update!
The issue is from your network provider. I have two sims in my phone, sim 1 ims is showing me not registered while sim 2 shows me ims registered. Moreover, my sim 1 doesn't show VoLTE which means my network provider hasn't activated it from their end while sim 2 always shows VoLTE.
In conclusion, the problem isn't from your phone but from your network service provider which is yet to activate VoLTE for your phone model.
