First thing check the fitting of the aerial cable to the plug is correctly done with the solid copper core wire fully through the central core.
If you are happy that the cable connection to the TV is good you could if the TV is not too big and heavy move it to a room where one of your other TVs is and connect it and try to tune to see if you get the full range of channels.
To determine which transmitter your TV should be tuning to enter your address details into the Freeview website here,
https://www.freeview.co.uk/corporate/detailed-transmitter-information
Based on your location the results will give you the transmitters broadcasting in your location.
The first listed will be named ‘most likely’ scroll further down and take a note of the multiplex numbers it broadcasts, these numbers are under the black circle with a white N inside.
Depending on your particular situation you may need to tune to an alternative transmitter, for example in my case my ‘most likely’ transmitter is 20 miles way but due to buildings it is better to tune the TV to another transmitter 40 miles away which has better line of sight and therefore gives my TV much better signal.
If you can determine which transmitter best suits your needs you can then do a manual tune to the chosen transmitter using the multiplex numbers (there are only five) it broadcasts on.
Before manual tuning take the cable out do an auto tune see if that clears the channels.
Cable back in and do a manual tune.
