C3700 N600 router - 2.4GHz stopped working (5GHz and wired are fine)

I have a C3700 N600 router that up until today had been working fine.  My 2.4GHz connection was spotty at best, and when it did connect was at least an order of magnitude slower than normal (and than much slower than 5GHz or wired ... and this test was done directly next to the router with only a single connected device).  After resetting the user and password and seeing no change I did a factory reset ... now the 2.4GHz will not connect at all.  I get a constant blinking light on the 2.4GHz LED on the router - yet no device will connect to it.  All the while the 5GHz and wired still work fine and at full speed.

So as a summary - 2.4GHz won't seem to allow any device to connect (yet will show as "attached devices" when viewing router setup).  Have tired renaming and changing password on 2.4GHz, and have tried factory reset.  Anything else I shoudl try?