I had the same issue with my samsung smartthings wifi mesh hub running plume software. no way to force 2.4ghz in the app. Plume support page has a solution:
Method 2 worked for me!
Method 2 (Android 7 or higher)Download and install Wifi Analyzer by Farproc and the Wifi Connector Library (both free) on your Android deviceForget your Plume network on you Android deviceOpen Wifi Analyzer and swipe right until you get to the AP List page.You will see several pods broadcasting on different bandsLong-press on the strongest one broadcasting on a 2.4 GHz channel (1,6,11) and choose Connect.Enter the Wi-Fi password and your phone will now be connected on 2.4 GHzUse the smart device App to setup your smart device.Once the device is connected, forget the network on your Android device and set it up again using the normal method.
Good luck! :)