Some comments in line below.
GLi-net have at least two versions of the AR750 available, including one with external antennas on the 2.4GHz radios.
The device have a 2T2R MIMO setup for the 2.4GHz radio and a simple 1T1R setup for the 5GHz radio.
Note that the antennas are limited to a few dB of gain (either external or internal) so you can't expect to get long range performance unless you have good clear line of sight between devices.
So it is a fairly basic WiFi router.
It does support both USB and SD Cards for additional devices and extended flash memory, which is useful if you want to share files e.g. for a digital library.