Hello,
On RUTX12 there is a useless SIM dropdown right now and it will be removed in future firmware updates because there is only one possible SIM card in the modem to choose from.
Regarding your issue on why you do not have a connection in the secondary modem, I suggest upgrading your firmware version to the latest then try using custom APN settings instead of auto you are using right now. Also, double-check if this SIM card has not reached the set data limit.
Regards.