Route or bridge?

Hi folks,
I have two routers set up, both as routers right now, the one controlling my LAN and hosting CAPsMAN is pretty standard with a WAN interface on Eth1. it is a class C subnet and serves as DHCP, DNS gateway and firewall.
Eth1 is connected to a second router, an LTE unit, that has almost everything disabled other than the modem, bridge, its single Eth port and DHCP, to serve up a different Class C to the first router.

The latter is a less capable device and also some 200m away in a field, so I would rather not have it control the LAN directly.

Should the LTE unit be bridging so that the IP assigned by the mobile AP appears on the WAN port of my internal router? (assuming that is what a bridge would do in his case)

What is be best option? either of the above or something completely different? functionality split between the two perhaps?

I plan on deploying a load of IOT type devices that will communicate using MQTT, all be it almost exclusively locally. Should I be considering a local DNS server and if so can I do that in RouterOS L5

Cheers,
Al