How to setup Smart WiFi Extender for 2.4Ghz-only devices?

Sadly there are some exceptions. So how can this be done manually?

So you have a device that tries to connect via 5.0 GHz and you don't want it to?  That's different from a client that only has 2.4 GHz access.

When configured for "mesh" operation, the AirTies copies the settings of the Gateway its working with.  If you disable the 5.0 GHz band on the Gateway, it should also be disabled on the AirTies.

It’s not really a matter of want, it’s that some devices cannot maintain a good connection on the 5ghz band (Apple), so the need to be kept on 2.4. Disabling 5ghz is like cutting off all the taller devices off at the knees, and dropping AT&T back ten years in technology. 

So,

1) Back to the original question, how do we configure for devices that should only be on the 2.4 band?

2) You mentioned, When configured for mesh. So how can the AirTies be configured?  And can mesh itself be disabled?

When you follow the instructions included with the AirTies device, it discovers the gateway and syncs up with it.  If it automatically assumed the gateway-provided SSID, then it worked.

The exact methodology of disabling a band in the Gateway varies by the gateway model.  You start by going to http://192.168.1.254/ .  Then on mine I select Home Network, then Wi-Fi.  To disable 5 GHz, I would change the Radio Selection to 5 GHz Radio, then change Wi-Fi Operation to Off.  Then click Save...

5 GHz band off

I think it is possible to deliberately configure the AirTies so that it won't "go-mesh" when located with a Gateway, but I've never tried to do that and I don't know how one would go about that.  But people have seem to have managed to do it by accident.

I should also point out that if you're determined that you need to have separate access, you can set up different SSIDs for the 2.4 GHz and 5.0 GHz bands.

I have similar issue and question - Due to constant hopping of devices between 2.4 and 5 Ghz I prefer to give the two networks different SSIDs. However my ATT Wireless Extender (Airties 4920) then gets confused, and broadcasts my 2.4 SSID network name on 5 Ghz frequency. I tried resetting the extender and re-pairing it few times, but no help.

The 4920 only sees the SSID for the 5gh even though the gateway is broadcasting both 2.4 under one SSID and 5gh under another SSID. It the 4920 will not rebroadcast the 2.4. All lights are working correctly and has been reset and WEP connected pet At&t tech folks. They can't figure it out and were to call me back in 20 minutes. We all know how that goes, no call back. They are worthless. We purchased the mesh extenders at their insistence to fix coverage issues. They never solved the problem. They then insisted we create separate SSID names and that is when it went to heck. They wouldn't connect to the mesh extenders at 2.4 anymore. Hopefully someone knows how to fix this.