Android phone can't see 5Ghz network after Marshmallow update

I've just updated my LG G4 to Android 6.0 Marshmallow, and it now no longer sees the 5Ghz network on my RT-AC68U - only the 2.4Ghz network. My laptop has no problem connecting to the 5Ghz network. I'm running 380.57.

Any idea why the Android update may have borked access to my 5Ghz network?