Since both these models support LTE connection as failover. I would like to know if LTE connection can be used as primary if the customer doesnt have any other option i.e. only LTE connection available in their area?
I think the Cellurar is only for failover. If you don't have any kind of WAN connected I'm not sure if it's works or not. I couldn't test it yet. Maybe @PhilipDAth has more knowledge if this could work or not.
One thing how you can overcome this issue would be to buy a 2nd router and connect it before the Meraki. In this way you will have a portable antenna what you can place to the window for better coverage.
Hopefully this helped.
The internal modem CAN be used by itself. The moment you plug in something into WAN1 or WAN2 though, they will become primary.
I tested it with MX67C and only using the Internal modem (warm-spare too) and it works, and fails over (slowly).
@NolanHerring is correct. This document has a section explaining how to use 3G/4G as the primary uplink for a device:
As mentioned it is highly recommended to use a WAN connection for the initial setup/configuration of the device before using Cellular as the main link.