Login to i.mi.com and unlock your device, then remove your device from Find my Phone. Even then it might take many tries for it to accept the login on the phone.
The SIM settings on my RN3 running 12.1 are greyed out, it's still working fine, can send & receive calls & texts, but can't change my settings.
Edit: I'd frozen the "SIM Activation" app, unfroze, rebooted and ok again. Oops.
This is based on the beta build. Beta software should not be relied on to be stable, that's why it's beta. This version is based on Xiaomi's dev build, not the other way around.