The hardware list is updated by the community. If it isn’t there, doesn’t necessarily mean it isn’t support, just hasn’t been tried yet. Have you search the forums for E3131?
Before you buy anything, you could also find other modems in that family and see if they are supported. If they, then the E3131 may work.
If it doesn’t work, you can send the support output files to Support and MK may be able to add it. It will be a bit of a gamble, however. You could just buy the cheapest RB that has a USB port.
I have rb433gl with usb port and Huawei E3131and this solution worked for me without problems. Depend on which mode E3131 is (hi-link or normal modem).