Is this happening with any content you try to play, or is it only happening when you try to play something from your Watchlist or Continue Watching section? (Try playing something from one of the other sections instead to check).
It's usually a good idea when NOW TV doesn't play anything to try rebooting your broadband router first, and while that's rebooting unplug your NOW TV box. When your router is back up and connected plug your box back in and try again.
Tethering the box to a phone's data connection isn't a great solution really, due to the large amount of data the box consumes and the need for a stable throughput, do you not have a broadband option where you live? I think there's a good chance it is this causing your problem, if possible I'd suggest going to a friend or family and setting the box up on their TV, connecting to their broadband to allow the box to update and see whether you can play content there.
@Anonymous User please note I have unlimited broadband here but when I changed suppliers I asked for a usage amount. I had been using over 100GB per month of data. I don't game or do large file downloads so my usage was down to Now and Amazon (evenings and weekends) I wasn't using it every evening or all day on weekends either.
Are you on either Giff Gaff or Three unlimited tariffs?