@Kamome reply is pretty much all of the options.
Would i be correct in assuming the SIM you have been provided is by your MPLS provider, so your data connection, even through via SIM, is being MPLS'd if you will excuse the expression. You arent going out on a general SIM card to a data connection and DMVPN or some such back into the MPLS?
The "C" series of Meraki routers are likely to be the best option, that and the MG devices. They will give you manageability of your WAN.
Get a trial of an MG and your desired MX and test it out. Get yourself lined up with a test SIM card from the MPLS provider and simulate it. You have 30 days for the Trial, no commitment.