It can take up to 14 days for the broadband speed boost be applied.
Info below from https://www.o2.co.uk/virgin-media-o2
Applying your Volt benefits
If the name on your Virgin Media broadband account matches the name on your O2 mobile account, your Volt benefits will be added automatically within 14 days, unless you were already a customer of both before 18 October 2021, in which case you'll need to go into My O2 to activate your benefits.
If your broadband and mobile accounts are in different names, or the names don’t match (eg Chris and Christopher), you’ll need to activate your Volt benefits in My O2, and they’ll be applied within 14 days.
You'll also need make sure the address on your O2 account matches the address on the Virgin Media broadband account, whether it’s in your name or someone else in your household. If they don’t quite match, you can edit your address in My O2.
If you’re unsure, you can check in My O2 seven days after you’ve placed your O2 order. If you think everything's correct and still aren't seeing the options to activate your benefits, give us a call on 202 from your O2 mobile.
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Sorry to hear your Volt benefits haven't been applied to your Virgin account yet, can you confirm if it's been over 14 days since you registered for this?