@chtsarts 

You can't turn off just WiFi 6 in Deco.

WiFi 6 just like WiFi 5 runs on 5GHz band. Devices which need WiFi 5 only will run just fine when WiFi 6 is enabled.

WiFi 4 needs 2.4GHz. Older devices may not be able to understand dual-band configuration, where one SSID of Deco Main Network runs with both 5GHz and 2.4GHz bands.

There is workaround you could try. Enable Deco Guest Network and configure it to run with its own SSID and on 2.4GHz only. For most oder devices that should be enough to have them connected to Deco mesh.

Newer Deco mesh firmware introduces IoT Network which is specifically tailored to older devices (it only runs on 2.4GHz by default), but your X10-4G may not have that firmware update yet. IoT Network is descrived here, and you can check in Deco app if your Deco mesh does or does not have that feature: https://community.tp-link.com/us/home/stories/detail/501922