To me it looks like either the device has somehow glitched from the unlocking process or it's not working in the U.K due to it being imported from the U.S to U.K if I've read that right about the 3uk SIM card.
If so then typically a network will say they cannot guarantee an import will work in the U.K so they can't support it.
A Samsung Experience Store or Samsung Service Location can run a diagnostic on the device which is something I'd suggest to take advantage of.
Daily Driver > Samsung Galaxy s²³ Ultra 512Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
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