Welcome to Vodafone Community
Let's try and get this resolved for you.
There was some cases recently where 4G hadn't been activated on a persons account so a person needs to contact customer services on 191 or Live Chat to get that done.
Just just ask a few questions if 4G is found to be activated on your Vodafone account.
1. If you insert your sim card in another phone do you only still get 4G ?
Can you try another sim card in your phone ?
If you get 4G from another phone then Is 4G 100% turned on in the connection settings in your actual phone itself ? Dis you ever receive a 4G connection using that sim card and phone.
4. Do you not get 4G anywhere you go that's supposed to have 4G cover or is the issue in one particular area ? If one particular area then the mast might be at issue or the area isn't 4G. You can check via inputting your postcode in the Vodafone coverage-checker.
Vodafone also have help in the Network-issues-initial-checks-and-template.
5. If all looks Ok with your Account and Phone then sometimes as a very last resort having the sim card replaced via a Sim Swap via a Vodafone Highstreet Store can help. Take some Photo ID to prove who you are or if you have no ID then be prepared to answer some security questions too.
6. If the Payg number isn't too vital to you then your extreme option could be to bin the sim card after using up any allowances and any rewards points if relevant and pick up a new sim card and start again.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.
This is certainly something easily solved for you.
Please see this previous thread: https://forum.vodafone.co.uk/t5/Pay-as-you-go/4G-not-working-on-new-PAYG-sim-card/td-p/2601779/page/...
The reply from the last poster may help with the issue you are experiencing. If going through the Customer Service channel does not help, give the forum team time to get to the thread and they will add 4G to your account.