Is the bmw connected tied to a particular service provider, or can you link it with whoever has best signal in your area (EE for us)
It is tied to a single provider, all the documentation says T-Mobile so I assume it uses the EE network. It's fine in my area, but I do drive my car outside the local area like most people I imagine

If the OP's Dad is going to get confused/annoyed by this kind of thing then its not a workable solution.