Signal issues

This will be down to the Marshside mast being removed at the landlord's request, putting pressure on other nearby masts. It has affected Three (Smarty's parent network) and EE. Getting permission to build a new mast is a slow, difficult process and is often refused due to local objections, so it may be many months before normal service is resumed.

While 5-12Mbps is slower than the speeds you're used to it isn't bad for mobile internet and should be able to manage most things without problem.

Do you have WiFi at home, to use WiFi Calling? That will solve the issue with phone calls.