Hello,
I had the same question, so I asked MikroTik. They told me the SIM socket is used for other products that use the same enclosure, but is non-functional if you have the LoRaWAN module. They recommended the RBLtAP-2HnD (https://mikrotik.com/product/ltap) for a device that can accommodate a LoraWAN gateway and a cellular modem in a single box. (I posted the same answer in the MikroTik thread MikroTik LoRaWAN gateways and concentrator boards).