Not connecting to mobile network

my iPhone 8 will not connect to my mobile network which is the phone coop (who use EE’s network)

i get this error:

i have updated it and had the hardware fixed. Now it will sometimes work for about 5 minutes after restarting the phone or resetting the network setting. Then show this error again.

Other SIM cards work on the phone and my SIM card work is some other phones. Currently in an iPhone 6 that works. But it didn’t work in an iPhone 7 at the phone shop. I’ve had the sim replaced too and that hasn’t worked.

Any ideas?!