All the 4G routers use mPCI-e modules that have u.FL connectors. You can easily add a u.FL to SMA pigtail and have your antenna.

The Mifi has drilled holes you just have to punch out for such a setup.

On the x750 Spitz, both the external antennas are for the 4G, the wifi antenna is on the board for 2.4ghz, and has a small internal antenna for 5ghz.

For the new Mudi, you would have to drill a hole for an SMA antenna, but you can use the same pigtail setup.