3G to 4G on iPhone 12

In UK I’ve just bought a new iPhone 12 to upgrade from my older IPhone 6S.

The new phone will not default to 4G whereas the old one does.

I've tried the Airplane mode off-on trick without success.

I've shut down the new phone and restarted with no success.

I’ve checked that the phone is enabled for 4G.

Any other suggestions?

or is it because new phone has a GifGaf Sim whereas older one has an O2 Sim?