Just kept connected me to Red Hill mast.
You need to read up on how cellular radio communications work. You aren't "put on" a certain mast, your phone listens to all the tranmissions it can "hear" and it transmits, and not all masts can hear your end. The ones that can hear you, negotiate with your phone to make a connection to the mast that gets the best quality signal (not necessarily the fastest!).
This is how mobile phones can work when you are moving, even at high speed. (e.g. on motorway).
Assuming the masts are active; they may not yet be integrated into the network so not properly engaging in this process.
You may just have to wait, or at worst, fit an outside antenna on your building.
24 years of broadband connectivity since 1999 trial - Live BQM