It isn't the box that's dodgy, it's your TV signal. An 18% signal quality reading is dreadful.
A good quality signal would give readings like the ones quoted by @TimCurtis . In particular, the signal quality reading should be 100%.
Consider employing the services of an aerial specialist to sort out the problem with your aerial.
You dont need a aerial to watch tv through a pro box.
You can stream content through your Internet connection.
Yes but not all channels and can't FF through ads on ITV to mention just 2 downsides.
Good to hear.
Out of interest check the quality & strength figures on channel 14 and 101 & 103 just to rule out any aerial issues and post back please.
Then that's the problem, your signal strength is way too high as that'll be overloading the tuners.
Get a variable attenuator (about £5-6), connect between the aerial cable and the aerial input of the box and adjust until the strength drops to 85% ( as a start).