What nowtv device are you using, a white, black, smart Freeview, smart 4K or smart stick ?
The Demand 5 App seems to be playing fine on my nowtv 4K smart box.
Have you tried some of the basic stuff, like reboot both the router and nowtv box or stick by disconnecting the power for 30 secs.
Check for any software update on your nowtv device by going into the settings menu.
Removing the Demand 5 App and then re install the App again from your box / stick Apps Store.
Just dug out my old black nowtv box and Demand 5 is streaming fine for me over Ethernet on NowTV Fab Fibre.
Wonder if it's possibly an ISP router related conflict when using the Demand 5 App on NowTV boxes or stick ?
Out of curisoity who is your ISP to see if there is a pattern emerging or not ?