As you know, class 12 has 4 slots for transmission and also 4 slots for reception, although only 5 slots can be used at a time. Is the modem capable of operating with 1 slot for Rx and 4 slots for Tx?

Our modem can only support Class 10 in Edge mode.
Class 12 is only supported in GPRS mode

There are no settings to configure the slot usage in the modem. It is up to the wireless provider to assign the slot usage and the modem will auto negotiate.