Well, let's turn this thread upside down. I just spent 2 hours trying to get my Rachio to connect to a 2.4GHz network, and it never did.
I created a new 2.4GHz only network (Wireless-N) using an ASUS all-in-one pocket router along side my OnHub network. Rachio wouldn't connect to this network.
I created a new 2.4GHz separate network on an ASUS RT-AC87U along side my OnHub network. Fail.
I took down the OnHub network completely, factory reset my RT-AC87U to all basic settings with disparate 2.4 and 5GHz networks. Fail.
And a bunch of little trial and error in between, each with a WiFi reset and reboot of the Rachio in between each of my tries.
Unless I am just dense and doing something completely wrong...I think my unit is bad.