Virgin Media's Essential Security service requires their DNS be used. To determine if this is the case, from the device on which it is not working, go to https://one.one.one.one/help/ and confirm the reported AS Name value is Virgin Media. If AS Name is not the expected value then that would explain why it is not working and you would need to investigate whether DNS is being set on Wi-Fi router and / or device; the former can be ignored if only using Virgin Media's Hub.
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