it seems to me, we use different HW versions of the RUT230 - RUT23001E000, Hardware Revision: 0010, Modem Model UC20. On my devices there is only one antenna for mobile communication.
On none of my four RUT230 devices the "connection method" shows up on the Network->Mobile page. There is only the "Service Mode" selector with "2G only", "3G only" and "Automatic". Every setting results in the same behaviour.
I updated the firmware from RUT2XX_R_00.01.11.1 to RUT2XX_R_00.01.13.1, (s.o.) but the problem persists.
The RUT230 devices are mounted in a steel cabinet, so I attached an external antenna with 2 m low-loss cable. Disconnectiong the antenna does not change the behaviour, the RUT 230 connects using the 3G mode, only the signal strength decreses down to -113 dBm. It prefers 3G network as log it is visible above the noise level. The much stronger 2G network is ignored.
The antenna cable is tightly attached.
Another SIM card from the same provider shows the same behaviour.