Whoah. Not only does using AP-WDS mode on the LAN wifi help with these disconnects, it also seems to stabilize the connection when using the 5G radio. I don’t completely understand why though.
Once I got the 5G driver installed and the radio up with stock connectivity, here’s the weird behaviour I would get from my AR300MD (I also got this when using the old 3.03 firmware and it frustrated me to no end):
Upon client mode connection to the WAN 2.4Ghz WiFi, luci would either randomly add two wireless connections (both seemingly with the same settings), OR it would add one new connection, but change the display of the SSID and Mode of my LAN connection to match that of the new WAN connection, thus killing my LAN WiFi. In the case of the duplicate, if I deleted the second one, the first one would remain, but then fail to connect to the WAN WiFi altogether.
For whatever reason this never happened prior to installing the 5G driver even though what I’m describing here technically has nothing to do with the 5G radio yet.
After reconfiguring both the 5Ghz LAN connection and the 2.4Ghz LAN connection to AP-WDS mode (rather than just AP mode), I have yet to see either of these problems and I’m able to successfully connect and maintain connectivity with the 5Ghz and 2.4Ghz LAN all while the WAN remains operational.
Thanks for the tip @blue-wavenet!