Disable SmartSteering in the wifi settings on the Actiontec. That will create two networks. One 2.4 and one 5GHz.
You can set a password for each. You can have them the same if you like. Just use slightly different network names so you can tell which is the 2.4 vs 5.
It will really depend on the smart device and the app used to control it. Some will work across separate networks in the home. Some will require the phone to be on the same network as the smart device. There are so many different devices out there it's hard to give an exact answer.