How can I choose a value plan on iPhone when mobile site doesn't allow me to click ok? | Technical service details | Consumer forum

I'm using an iPhone and safari to log in to my account. I was shot to top up, but when I try to add a value plan a window pops up where I need to click (I'm assuming) okay but the pop up is huge and I can't scroll down to click the button.

When I scroll down it scrolls the page in the background, not the pop up.

I can't find a work around and it's annoying.