As I see it, you have several options:
1. Go onto chat and ask if they can sell you a replacement remote.
2. Buy a Logitec Harmony remote, which can replace 8 or 16 remotes depending on the model.
3. A one-for-all remote might work, but I don't have any experiance of those.
4. I'm sure someone else will pop along with more suggestions.
Have you got any friends or family members who have either got the white or black nowtv box where you can borrow the remote controller just to setup your box on your new wifi home network, then you can revert back to the Roku remote controller app has before ?
Welcome to the forum.
There's two ways round the problem without resorting to finding a remote.
The easiest is just to change the wireless network name and password on your new router to match the name and password that you used to use on your old router, then unplug the NOW TV box for a minute. When you power it back up, it will connect to your new router thinking it is still the old one. Most routers will let you change the wireless name (sometimes called SSID or wireless SSID). Check the info that came with your router and/or contact your Broadband helpline if you need help logging into the router and making the change. Also bear in mind once you've made the change, you will need to update your phone and other wireless devices to the new name and password as well.
Alternately, if you still have your old router, unplug your NOW TV box, then power up the old router (just the power supply, don't connect the telephone cable to it) Wait a few minutes and then power up the NOW TV box and it should connect to the old router as normal. Then change your phone wireless settings so it also connects to the old router. Obviously there will be no internet connection but because your box and remote app are now connected to the old wireless network again you can use the remote app to control your box. On the NOW TV box go to
Settings > Network > Set up new wireless connection
and then set up the box to connect to your new router's wifi network. Then set your phone back up on the new network as well and away you go. You can then unplug your old router and put it away again.