Even if Now TV Box does not permit to change DNS you can still use SmartyDNS service on it. You just have to set-up DNS on your router, so it will take effect to your Now TV Box as well.
Here are DNS tutorials for most popular routers: D-Link, TP-Link, DD-WRT, Asus, Linksys and Netgear. If any problem occurs during set-up process you can always appeal to our support team by opening a ticket with information about your router and what problem you have.