I topped up online just to get credit for my emergency Nokia phone
Which provider did you do this online top up with? (Remember that Nokia is your phone, not your PAYG service provider). When your phone is switched on, you should see the provider name on the main screen.
Years ago my old Nokia flip-phone used
T-Mobile for it's PAYG provider. As and when needed, I'd purchase the T-Mobile top-ups from the customer service qiosk at my local Sainsburys for £5 and £10 each. The top ups looked like ordinary till receipts. I'd enter the PIN code (displayed on the receipt) onto my phone. I rarely made calls, so my top ups usually lasted for a couple of months.
Important: You can't 'mix and match' service provider PAYG top ups. In my case, I had to stick with T-Mobile top ups.