NowTV box won't need to be in DMZ. What DMZ does is allow other devices on the internet to connect inbound to it - often used for multi-player gamers for XBox, PC, PS3/PS4 etc.
The NowTV box works the same way as any other video streaming box, and connects out from your internet connection to the website. So no DMZ required - if it isn't giving a picture at all, then its either the wrong cable, or incompatible with your TV.
plusnet unlimited fibre
80/20 since 2 Jun 14 - Sync as of 7th Aug 16: 55,355/10,291 kbps with G.INP
17 years of UK broadband since 1999 ntl:cable modem trial -Router: Asus RT-AC68U with HG612 - BQM