Outdoor Version of MR1100

"There are units on the market (eg Yeacomm), but they are not Nighthawk!"

There are many, much better, options for your scenario than Nighthawk.

While I value the MR1100 for it's compactness, and battery life - it is a poor choice as a 24x7 mobile router.

Even though it is 4xCA, it makes poor choices about it's connections (probably due in large part to AT&T's load balancing algorithms), and is not very stable.  Additionally, you need to have a degree in programming to enable bandlocking (which, officially, can't be done - so no support there).

I'd also like to say, I've NEVER seen my 2A1NAS connect to more than 3 towers, even when in a "target rich" environment.

Enabling bandlocking does help tremendously in obtaining bandwidth, but in my experience, that bandwidth falls off rapidly with the MR1100.  So, practically speaking, I can start to stream a movie, but I may have to unlock and re-lock bands a dozen times while watching the movie.

I don't have that problem with the more commercial oriented hardware I use at my home - which is mounted 50' up a tower beside my house...

For a complete solution more suitable to your situation, you could Google "Outdoor LTE PoE Carrier Aggregation".

I would look for at least 4x4 MIMO, 4xCA (or higher), and verify bandlocking is supported.

Alternately, if you would prefer to roll your own, you could Google "Outdoor Mini PCIe Enclosure Antenna PoE", 

and "Carrier Aggregation Mini PCIe Modem".  Some assembly required, but you can really up the ante in specifications an performance.

I see multiple enclosures with provisions for 4 or 6 antennas, and PCIe modems with 5x and 6x CA....

"Yes, outdoor antennas can be used with the Nighthawk. However, the signal loss in a length of coaxial cable often negates the gain from the antenna."

Should you remain fixated on using the MR1100, you could invest in a cellular repeating system to relay signal from an outside tower to an inside area.  It is in this area that you would position the MR1100.

This scenario has a hidden advantage.  The MR1100 is actually a 4x4 MIMO, meaning is has 4 Cellular antennas internally.  The rear TS9 antenna ports (Antennas 1 & 2) are for 700MHz cellular bands.  Antennas 3 & 4 are for the 1800MHz to 2.7GHz bands.  Almost without exception, these higher frequency bands will have the greater bandwidth.

If anyone is curious, all four antennas have uFL SMD connectors hidden inside the MR1100 housing.