We have been using RB715G-2HnD (v5.20) for a while now with great results. Before deploying them into our locations (we are a QSR) we want to utilize external antennas because the routers will be below counters and we feel as though reception will improve. We have procured antennas (like this one http://www.roc-noc.com/mikrotik/routerboard/antenna/indoor/omni/swivel/mmcx.html).
I am unsure what I have to modify in the configuration to use the antenna. From another post I tried the following:
deactivate chain0 tx, activate chain0 rx, both chain1 rx,tx activate and select antenna b.
When I tried this it knocked out wireless completely.