UK cellular backup - only one bar on EE

Hi, folks. I'm new to Ring with two weeks remaining of my trial. I'm concerned that the cellular backup of my system relies on EE (the poorest cellular signal in my area) which shows only one bar no matter where I position the base unit in my house. It would appear that EE is the default cellular back up with no option to change to either 02 or Vodafone, both of which offer excellent coverage in my area. Is there anything that can be done? I really like the system and have invested heavily but am conflicted about the lack of EE cellular signal for obvious reasons. Grateful for any advice by return. Cheers.