I need to set up multiple LTE WAN connections (US-based) and I'm having a hard time figuring out what would work best.

I see two products that look like they would work:
RB912R-2nD-LTm&R11e-LTE-US and RBLtAP-2HnD

I don't need an AP built-in but it seems that comes with all of the products.
RB912R-2nD-LTm&R11e-LTE-US has two SIM card slots, does that mean I can have two separate carrier connections at once? Same question for RBLtAP-2HnD, but it has 3 SIM slots.

This will be for inside a data center so I really just need a modem and router.

thanks!

First,
Install at mobile the CellMapper and check what Band it detect at your area and with your ISP.
Check website of CellMapper, the MAP and what Band your ISP use.

Next is check that modem's supported Band's and then you can thinking about what RouterBoard use with modem.

I think the MikroTik not have a proper modem for USA, like Band's 2,4,30,12,66 at one modem (example from AT&T).
I know that Telit LM940 have got more of them.

Please learn about LTE before buy and install hardware in many places.