a51 prob.

[Frankoe]

Anyone know why I keep getting a " Not registered on Network" message even when I have 2 bars? I spent 2 days this week unable to call or receive!!

[Meh90]

does the sim work in another phone?

in the mobile network tab in settings try to change network mode to 3g only and back to 2g 3g 4g auto.

also
network operators tab which should be on automatically. disconnect and reconnect.

If all above does not work, contact your mobile provider directly.

[BandOfBrothers]

This is usually a sim card issue @Frankoe 

Eject the sim and then ensure it's seated correctly.

Your Network store can sim swap to replace it.

Sometimes a sim card can work fine in one phone but not OK in another phone !

I wish you all the best with this situation.