hello,
i'm having trouble to get my "ALFA AWUS036ACHM" to work on my GL-x750v2
can anyone help? Not working means no lsusb visibility nor anything i could conclude it is even connected or recognized by the router. What drivers/where from should be installed i though this device is supported without installations.
Do i need to activate the USB port first? lsusb does not show anything.
Thanks 
is it discovered in dmesg ?
if not, then there's something wrong with the adapter or the port.
USB-devices do need drivers installed, yes. It'd increase OpenWrt's size WAY too much to install all such drivers by default. You need to figure out what chipset your USB-device uses and install drivers accordingly.
I'd also check, if your router's OpenWrt-image includes drivers for the USB-port itself. As far as I know, most images have those built-in, but I obviously haven't checked every single image, so I can't say for certain.