Combine 2 Wi-Fi Bands

Normally the Gateway comes configured out of the box with band steering enabled, meaning both bands have the same SSID address and your device can decide which band to use.  I would interpret this as "combining them".

Many people have issues with this and want to give the two bands different SSIDs.  This I would call separating them.

If you want the former, you should already be set.

If you want the latter, go to http://192.168.1.254/ , click on Home Network > Wi-Fi, enter the Access code from the side of the Gateway when prompted, then click on Advanced Options.  From there you should see separate sets of settings for both the the 2.4 GHz band and the 5 GHz band, including the SSID.  You may change one or both of these [warning, if you change both, things connected to the old SSID will lose connection] and click Save.  At that point the Gateway will warn you about the end of the world, a lack of chocolate in your diet, and possibly Mercury going into retrograde if you proceed.  You may ignore those warnings and proceed.

By default the AT&T gateways have both bands combined already.  If you only see one network name (SSID) when you try and connect a wifi device then that's how you are already set up.

I'll take issue with what you were told, however.  Yes, that's the theory but the reality is that many devices have trouble dealing with the combined bands (called band steering) and can actually make things worse.  All you can do is try it both ways and see if your particular environment and devices do better with it combined or separated.

A slightly different approach would be to see which of your devices are 2.4GHz only and point them to the Guest Network, which is 2.4GHz only. Give the Guest Network a different SSID (and pw if you want) and see how that works. Your 5GHz devices should stay connected to the 5GHz band because you didn't change the SSID or the password.