Are you using the usb port off your TV to run the nowtv stick ?
If so, what happens if you use the supplied power adapter instead, does the stick still lose the wifi connection on your router when you switch your TV into standby ?
I don't use a nowtv stick in my setup, where i tend to use a nowtv box wired over Ethernet.
Maybe try moving your router further away in your room from your stick, where having the router so close within 1 metre might not be the ideal situation.
If your router is dual band try using a wifi channel on the 5 ghz frequency band to see if the wifi is more stable without losing the network connection,
I think Im experiencing more or less exactly the same issue OP.... I've been having a few problems with this stick, only bought it Black Friday.
USB powered from the TV and wirelessly connected to a 2.4GHz hot-spot. A few variables to test then?
I generally have issues and a "no wireless connection error" when switiching my TV on but I've just been watching the live darts and I lost Now TV/wireless. Had to watch on my PS4. I've tried stick factory resets etc. but the problem remains.
Will test some things out later.