There are two places in GUI to set the preferred service type, one is per SIM card (Interfaces → SIMs) and one is for every WWAN connection (Interfaces → Mobile Interfaces).
As soon as the user sets up a WWAN interface the preferred service setting of that WWAN interface will be used regardless of the SIM setting.
For example setting SIM to 4G only and WWAN interface to 2G only will result in a EDGE connection.