Suggestion for external antenna

I'm sorry I don't have a good recommendation for an antenna.  Performance of the MR1100 with external antenna(s) has been a bit hit or miss with a lot of trial and error.

You would likely do better to mount the MR1100 outside (in a weather proof plastic enclosure, in the shade).  For example, on the North face of your home, under a small overhang to protect from direct sunlight.  Turn off the Wifi, and run ethernet to an indoor Ethernet switch and wireless access point.  Or perhaps in the attic?

This way the MR1100 can make use of it's multiple antennas for CA and MIMO.

If you must use an external antenna, you still want the MR1100 to be as close to the antenna(s) as possible and use high quality cable (LMR400) for runs over a few feet.  

The jury is still out on whether one or two antennas is better.  Antenna should be cover the frequencies used on your local tower.  Antenna should be directional, and aimed at the mast.

There are other more robust solutions as well like a Mofi or Cradlepoint Cellular Modem.  But these are more expensive than the MR1100.