How to use pay as you go credit towards my monthly bill

I uploaded 20 pounds via top up option. Still the monthly amount was deducted from my bank card.
How can I use the top up balance towards paying monthly cost.

Thanks

Similar problem, can I transfer my PAYG credit to my monthly plan?

Thanks in advance.

PAYG credit can't be used to pay your monthly plan; this will always come from your bank. This credit is for covering extras not included your plan, such as phoning foreign countries or calling non standard premium numbers etc. Also, if you run out of calls, texts or data before your plan renews, the cost will be taken from the credit. If you think you're not going to use the credit for any of this, you can always contact Lebara and ask them to pay it back to you. I did this a little while ago on some I had accumulated.