UK poor cellular signal

Hi, The signal strength of mobile backup is poor. It's using EE and only has single bar. I've read that Ring has contracts with all UK mobile networks and that the strongest signal is automatically selected. Vodafone is the best signal in my area. I phoned Ring support but they had no idea what I was talking about. The person I spoke to said it depends on the sim I have in the base station. Is there anyway I can get the base station to change mobile network?