I would avoid anything with less than 16 MB of flash or less than 128 MB of RAM.
GL.iNet has a unit designed for commercial, mobile use (GL-X1200 "Amarok", the black one with 7 antennas in some photos on their site), but I don’t know your price range, nor its single-quantity pricing.
Edit:
Unless physical constraints prohibit it, you might want to look at a card in a case with USB 3 connection to another unit. Most of the all-in-one LTE units I am aware of are single-core MIPS-based SoCs, not multi-core ARM. While probably adequate for LTE speeds, single-core MIPS-based SoCs typically struggle with high-rate connectivity (over a few hundred mbps).