@Shaw What was the error message?
You didn't put your account on Lost/Stolen, right? If you did , you have to reverse it first before you can pay. You go to go back to Lost/Stolen page and Resume service.
For payment, give it once more try using "Other (Enter the desired payment amount)" and then manually enter the plan amount . This usually work better than "Amount Due"
If that work and you are able to pay, then click the button Reactivate current plan if the button is around.
Then logoff My Account and reboot your phone