Have you investigate a data bolt-on?
That can be applied immediately, as far as I know, to PAYG.
Are you already on a Big Bundle?
If so, other than buying a data bolt on as suggested by @pgn, then you can only top up on your anniversary date.
Also customer services are closed today and open from 10am tomorrow.
*The Game Is On*
Payand Go was closed Saturday and of course closed today
Everything is taking longer due to the lockdown. Call centres and social distancing means limited staff to handle all the queries. Patience needed I'm afraid.
Other numbers to try
0800 977 7337
0800 587 4005
Track usage, upgrade and get support with the My O2 appDownload My O2