It looks like it fixed itself after I spoke to support. 
I booted up an older Android phone that has a 2.4 GHz only Wi-Fi band and then proceeded to install the app, thinking I will have to factory reset the Wi-Fi settings on the Rachio unit. To my surprise, after the app was installed and I started it, I noticed the controller was already online and in standby mode. Not sure if it joined itself as a result of me installing/launching Rachio on an older phone, or it somehow figured itself out prior to this.
Strange.