If you are asking about forcing WiFi clients (not XT8 node) to the 2.4 GHz SSID band. One way would be to disable Smart Connect (on the Wireless -General page), setup independent/unique 2.4 GHz, 5 GHz and 6 Hz SSID's with their own independent/unique passwords. Then connect the clients you want to the 2.4GGz SSID (and the rest of the clients to the other SSID's. Assuming of course one has Smart Connect enabled initially.
PS: I don't use Smart Connect so I cannot comment if the following is one way to force WiFi clients to use the 2.4 GHz band. Smart Connect has a Smart Connect Rule option where one can adjust and configure Smart Connect. Among the options is the VHT option for each WiFi band. The VHT option appears to let one select if certain client devices based on their hardware capabilities can connect to that WiFi band. More on Smart Connect here:
Or I suppose one could try using the Wireless MAC Filter option to block or allow certain clients on the 2.4 GHz or other bands. (MAC spoofing is possible as a way to bypass a MAC filter.)