Extremely slow internet speed on brand new mobile contract

I recently activated my new O2 mobile contract. Yesterday, my online overview showed "Unlimited surfing#ws" and "A day remaining in the billing month".

Last night, the internet started working perfectly at normal high speed. However, today it has suddenly become extremely slow again, even though my phone shows full 5G signals and the APN settings are correct ("internet").

Today is supposedly the payment/billing cycle transition date, but the high-speed data limit hasn't refreshed and I'm stuck on throttled speed. I checked my bank details and everything is correct, but no payment has been debited yet.

I have an urgent university assignment due tomorrow and desperately need my contract's high-speed data to work. Could a moderator please check my account status and refresh my data profile?