Ok, ok.... I tried all the steps in 'Help' and none of it worked as the button for 'add payment' was missing. This both on the web and app versions.
But, I noticed in the app version, my profile set up had gone from being completely full with all bars green, to one going grey and the rest being yellow. When I clicked on it it said to enter a new payment method, I clicked it and it loaded the screen. All sorted.
Quite convoluted.
So try that, and if you have an old credit card loaded, make it default, then remove it (again really convoluted). Who their iX team is for this is not doing their job