Coverage is variable across the UK indeed no mobile phone network guarantees 100% service / coverage..
EE have good signal in some places such as the peak district (as historically Orange) and diabolical in others such as in parts of Leeds, Sheffield and Central London.
Where have you been having issues? (post codes are useful)
Mast issues can take a while to resolve especially as they need to arrange access to where the mast is and in some cases request planning permission to replace it
The fastest way out is to pay off your device plan and cancel your airtime which leaves you free to get a SIM only deal from EE.
Track usage, upgrade and get support with the My O2 appDownload My O2