Set Ubiquiti Unifi AC MESH (UAP-AC-M) to station mode

After successfully installing the latest snapshot on a UniFi AC Mesh access point I am trying to connect it to an existing WLan network.
But that seems to fail.
Even though I was able to let it create its own network it seems not to be able to connect to an existing one.

Could this be due to some limitations of the driver? I can't imagine that the hardware should not be capable of doing this since it is a very basic task and the stock firmware supports the so called "Wireless Uplink" feature.

If it would be helpful if I provide further information, just let me know.