I know that cat 18 modem is embedded and not replaceable. For now there isn't cat 18 modem minipci-e from Mikrotik. For now 2 options:
1. Wait... Also see future 5G outdoor solutions. Maybe will be best than LHG18.
2. Replace your actual LHG and buy LHG18.
If you see LHG18 uses a Quectel LTE-A EG18 chip, but it not offers a pluggable card, only the chip itself that needs to be soldered directly on PCB.
https://www.quectel.com/product/lte-a-eg18-series
Regards.