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Is he in the same area it's worked in before @PSword
Does 4G not show at all in the iPhones status bar ? Or does it show 3G etc ?
Perhaps try a soft reboot by turning the phone off and on.
And check in the iPhones settings that 4g / LTE is selected still.
Also try resetting the network in the iPhones settings under settings , reset , reset network settings.
Customer service on 191 or Live Chat can help by checking his account to ensure 4G is still activated on his account.
Current Phone > Samsung Note 10+ 5G _256Gb > Model: SM-N976B.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.
Following on from BandofBrothers excellent advice, there are a couple of other things your son can do.
1) Turn mobile data off.
2) Go into airplane mode for 2 minutes and then turn off airplane mode.
3) Turn on mobile data.
If you have no joy with that, try to go in network settings on your phone and choose to manually find network. This may take a while (up to 5 minutes), as your phone is actively seeking out all mobile signals. When available, choose Vodafone. Depending on what phone you have, you may also have the option to choose 4G Vodafone in the list of available networks. It goes without saying to choose this if it comes up!
Good luck 😉