What happens if you use a different internet browser on your phone when going to the nowtv website to change your payment pin on your account ?
Maybe also disable your ad blocker at the same time.
If you are getting the same unsuccessful result, have you access to a different device with an internet browser such has tablet, laptop, etc to see if that makes any difference ?
I have tried all three browsers on my PC and on my laptop. There is no way to access payment pin that i can see on the app. 11 days in and no response on phone, chat, email, twitter and facebook.