I have the same setup on 2 RB-962 / hAP ac routerboards but for some reason the 2.4 GHz interface isn't working on the one I'm on now.
I've rebooted the router a few times but no change. My phone sees the 2.4 guest SSID but can't get an IP, this works fine on my home router.
The 2.4 GHz interface shows up as a disabled port in the bridge window.
My thermostat and other devices only have 2.4 so 5 GHz is not an option.
thanks