I've a strange issue which I can't find any reports on after days of searching.
I've had my Now TV box ( 4500SK model ) a couple of years with no real issues.
The last month or two my box will respond to the remote to switch on but then stop responding for roughly 15 minutes. I'll have to use an app to control the box in that time. Once the box has "warmed up" it starts responding again perfectly and has no issues again for hours of operation. After the box switches off again and I return to it use it after a period of time, the issue pops up again.
I've factory reset the box and uninstalled lots of apps, but the issue arises again. I don't think it's an issue with the remote as it works fine for hours and it has a new set of batteries. I think it's a strange glitch with the box
Unless it reported by other NowTV Freeview Smart box owners then it might be a glitch or bug, i can only assume it's an intermittent fault with the remote or the IR receiver on the box itself (unless it's some sort of interference where the box is located).
Have you tried a different position for the box or maybe temporary test the box on another television in the home (you don't need to use a TV aerial if the second television doesn't have one).
Sorry i haven't got an answer and just suggesting things that i would try if in your situation.
My own thoughts on your problem is that your remote has gone bad/faulty and should be replaced. I base my conclusion on the fact that your downloaded remote app, on your smartphone or tablet, still works regardless.
So that means your box does not have anything wrong with it's ability to receive instructions, which probably points to your remote losing it's ability to send instructions, albeit in daily 15 minutes gaps.
Replacement Now TV RCs are fairly cheap from Amazon, so it should not be a biggie to get a replacement.