Q: upgraded to 6.1.2 with wrong region

Not sure what you mean by "North American menu."  Region settings have nothing to do with connecting to a cellular network.

Settings > General > International > Region Format > set what you want.

If you lost carrier connection after updating, this usually means the phone was previously hacked to unlock. When you updated, the hack was removed and the phone was re-locked to its oringal carrier.  It will now work only with a SIM card from that carrier.

All iPhones are compatable with  all GSM channels.

Settings > General > Reset > Reset Network Settings.

What SIM card are you using?

Check with  O2 to verify that they still have you provisioned for international roaming.  The update may have screwed that up.  Otherwise,  you need a new O2 SIM card.  Updating has been known to fry SIM cards.

If you update outside the area of your home carrier, the phone doesn't activate with the roaming carrier and the card may not work properly again.  Have seen reports of this.

Life's a b*tch, and then you die.  Right?

The 5 iPhones I've owned have been just fine.  Sorry to hear your's isn't.