I have a Chromecast Google 4K TV with the supplied remote and the NOW App runs fine for me.
I don't tend to use it anymore, but that's another story.
I don't use the Chromecast feature on Google TV so can't really confirm if it's working or not when watching NOW content.
I do have Boost and don't get the commercial adverts (just NOW trailers) so I don't know if this is having any bearing (it shouldn't but I have seen a few different reports of glitches / bugs when not using Boost).
Do you have Mobile data on your smartphone, if so perhaps try temporary using your smartphone has a wifi hotspot (if it's possible on the Google TV) and delete the NOW App and download a fresh version from the Chromecast Google TV Apps Store and see if anything plays.
If the adverts wouldn't also play I would have said that it might have been a VPN or regional setting causing the problem.
Apart from the above the only other things I could suggest is unplugging the power to the Router and Google TV for a minute or two, clear cache or last resort undertake a factory reset on the Google TV.