The firmware on mine is 3.024, and it isn’t showing any updates available.
Mine is a different router from the one here: a GL-AR300M-337. But as my problem is the same I thought it might shed light on what is happening.
I’ve been through resets, etc., but as the router (in WISP mode, haven’t tried any of the others) works with devices other than a Chromebook without problem, I concluded the problem is specific to Chromebooks.
Thanks.