I thought I would share my experience with everyone about using an ac600 wifi dongle with a raspberry Pi. I have multiple Raspberry Pi systems ranging from the PI1 to a PI2 running version 8.2.5. I have to confess that I got very frustrated dealing with the problems I was experiencing. I even bought a new router, a tri-band EA9300 by Linksys, capable of handling 4k streaming data and my RPi2 was still having great difficulty connecting.
Whenever I would try to connect to one of 5G channels, the PI2 would refuse to connect. So in frustration I decided to try the 2.4G channel. I put in the passphrase and everything was working just fine. But 10 minutes later, Kodi indicated that the connection had dropped and when I tried to re-establish the connection, the passphrase field was blank and required me to re-enter the passphrase once again. I tried over and over again to enter the passphrase only to have to repeat the process 10 minutes later.
As it turns out, the drivers used by LE on the RPi aren't sophisticated enough to deal with a network with identical SSIDs and I have more than one router with the same SSID. I changed the SSID of the 2.4G channel on the new router and was then able to connect successfully without the connection dropping.
With that success, I decided to try the 5G channel again. Unfortunately, the same problems surfaced again. I then realized that the EA9300, a tri-band router, gave both of its 5G channels the same SSID. So once again, the drivers in LE had problems connecting. I turned off the fancy "5GHz steering mode" on the router which allowed me to name each of the 5G channels individually. I was then able to connect to any of the available 5G channels with no problem.
So the moral of the story is that if you use multiple SSIDs with Kodi boxes powered by a Raspberry Pi, make sure that the SSIDs are uniquely named so that you can avoid the conflicts I ran into.