@Tal-ash,

Mesh Systems are backwards compatible and should still be able to connect devices if they only support 2.4GHz. Saying that, some IoT devices still have issues connecting to the network due to differences in Wi-Fi implementations.

I would recommend attempting to connect your AC unit to either an IoT or Guest Network, as the bands can be turned on and off for these networks. If you are still having problems connecting, it may be a result of the wireless security settings for the network. Try using a lesser method for your IoT network, such as WPA /WPA-2 Personal

Are you able to see the AC in the list of connected devices through the process, meaning it can connect, just your phone cannot find the device for setup? If it shows in the list of connected or offline devices, there may be settings to change that will help keep the AC connected, such as Fast Roaming, Mesh, or Beamforming