Some EU laws apply in Northern Ireland - I don’t know about laws in this area specifically. If a phone is legal in the EU, the UK may be required under the Northern Ireland Protocol (and the new “Windsor Framework”) to allow it in Northern Ireland. If they have to make a phone legal in Northern Ireland, it will be very hard to ban it in England, Scotland and Wales

Sure, but its not that hard or expensive to hop a ferry/train/plane trip to France or Ireland and buy an unlocked phone, as one example. Any laws the UK passes have to account for the geographical realities of its location.

Just because the UK is no longer in the EU doesn't mean its not affected by many of its decisions, given the enormous volume of trade that will continue to occur between them.