I don't use an iPhone or iPad but see if there is anything in the settings that has anything to do with regions or countries.
Also maybe try deleting the NowTV App and go to the Apple Apps UK store and download a fresh version of the NowTV App.
Thinking about it now i believe i may have read on one of the help articles from nowtv (which i can't seem to find now under their Help Centre section) that a few customers was experiencing exactly this and i can't remember what the fix was.
See if live chat can help you out.
Open the link page below and select the most appropriate help topic, then once you reach the final stage scroll down to the bottom of your screen page and click the drop down arrow below Get in Touch.
Just found the article in question from the NowTV Help Centre.
This is the part that seems to be effecting you.
As your NOW TV service is only for UK residents, roaming abroad is only available for 30 days at a time. After that, you'll need to confirm you're still a UK resident by watching NOW TV back in the UK before you can watch abroad again.
After you've watched NOW TV back in the UK again, you're free to start another 30 day roaming period next time you travel to any of the eligible countries listed above.
We're aware some customers may experience issues watching NOW TV back in the UK after reaching the limit for watching abroad. We're working to fix this as soon as we can.
In the meantime, you can start watching again by re-entering your card details in Payment details.