No WiFi coverage in half my flat!

I’ve had Virgin internet in my flat for 3 years and have only ever had WiFi in the front 2 rooms. I can’t test WiFi speed in half the flat to show I have no coverage and need the WiFi booster set up done (for free as I believe as I have minimal coverage!)!!!! Is anyone able to give advice as to what I can do, as want to move the tv and internet to the back of the house now we have updated our kitchen and living room, but it will mean no internet at the front of the house??!! How do I fix this as I feel this is something Virgin Media should fix as I live in a ground floor flat (in a small terraced house) with only half ever getting WiFi!!