@-Paul- You're a genius sir! I too use Pi-hole on my LAN to block ads and for weeks I've been getting the "NowTV currently unavailable" issue. Their technical support people are idiots.
I whitelisted pubads.g.doubleclick.net in Pi-hole and although it didn't seem to work at first, after a couple of reboots everything seems good.
It *really* annoys me that NowTV require end users to allow advertisements or the device simply doesn't work properly! Especially given that the NowTV stick costs £25 and most of the content is paid for too.
UPDATE: actually, after another reboot, it stopped working again. It looks like you have to whitelist dpm.demdex.net
Issue was resolved for me by change to BT Web Project, allowing File Sharing access. Goodness knows what NowTV have done to require this change, which reduces protection for users.
All other solutions didn't work for me.
Using pfSense router and pfBlockerNG which is similar to pihole. I had to disable the DNS ad blocking, and restart the DNS resolver. Then unplugged the Roku stick and plugged back in again. I suspect the stick is caching the DNS responses from the ad blocker.
This resolved the issue. I could then re-enable the ad blocking DNS on pfsense whilst the stick was on and the stick continued to work as normal and other devices had their ad DNS queries sink'ed.
Adding the doubleclick and the other queries the stick was making to the whitelist, suggestions that have already been made, utterly defeats the purpose of the DNS sink and the device was being returned if it required these. doubleclickads and google are the single biggest ad providers on the net and whitelisting basically renders the DNS service pointless if you whitelist these.
As an update to the above, this would not work over reboot of the stick or if the TTL of the DNS query expired, so in the end I assigned the google DNS servers to the DHCP lease for the Roku MAC address and added allow outbound rules to the firewall for port 53/UDP to 184.108.40.206 and 220.127.116.11 from the statically assigned IP for the Roku stick. The fact that this is necessary for the stick to load the home screen is a joke. I got the NowTV stick as a temporary solution and it will stay that way. Note that this requires a relatively flexible router - any ISP routers that I have ever used do not have this level of customizability and I could not find a way to set IP/DNS config for the WIFI connection on the stick.