did you ever resolve this? i'm having same issue
I had this problem with my legacy pre-paid account. For one second it flashed the screen "There is no primary account holder" and then back to refill. I went through this at least 50 times or so, trying to figure out how to fix it. What fixed it was to use Edge instead of Chrome. It was a Chrome problem controlling pop-up screens. Edge was zero problem. Put in the credit card details and was able to refill my account. I am overseas and have had this problem with another phone company's website paying my landline bill. So try other browsers and see if that is the problem for you. It was for me.