All Virgin Mobile SIMs become 5G capable when moved onto the Vodafone radio network if in a 5G capable device and you're in a 5G coverage area.
Most SIMs should now be on the Vodafone radio network, but you can check this by doing a manual network search. If you see Virgin and EE networks, you're on the Vodafone radio network. If you see Virgin and Vodafone you're still on the EE radio network.
If you are on the Vodafone radio network, you can check your 5G coverage here https://www.virginmedia.com/mobile/coverage-checker/5g-care
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I work for Virgin Media - but all opinions posted here are my own