It’s really hard to say how you can use the USB dongle, without knowing what you have. First you need to figure out what kind of chipset is inside of that dongle, so you can use opkg to add the needed drivers.
After that you need to adjust /etc/config/wireless to add another radio section. You can use the command “wifi detect” to get the section you need to /etc/config/wireless . After that it should work like any other device with a 5GHz radio. In my case I used a Netgear WNDA3200, which works nicely!