Chamberlain MyQ doesn't work with ATT internet Air

The Internet Air Hub guest network is 2.4 GHz only. Maybe you could try enabling and using the guest network for your MyQ devices.

•     Take your web browser to http://192.168.1.254
•     Click on the Home Network tab
•     Click on the Wi-Fi tab (If prompted, enter the maintenance access code listed on the back of the hub)
•     Scroll down to Guest SSID
•     Change Guest SSID Enable to On.
•     Set Network Access to 'Internet & Home LAN' to allow communication with other LAN devices.
•     Set the Password (minimum of 8 characters).
•     Click Save...

I just tried your suggestion and when I connect the MyQ to the Air hub via ethernet, the MyQ still won't work. The MyQ has a green light that goes solid when I connect it to one of the ethernet ports on my old ATT cable internet box, but it won't come on when connected to the Air hub in either the 1G or 10G ethernet ports.  Your instructions were great. Thanks 

Does the LED on the Air Hub Ethernet port light up when you plug in the MyQ?

No it doesn't light up on the Air Hub ethernet port either when I plug it in.   When I plug the directv ethernet into either of the 2 ports, it lights up green on the port it's plugged into.

Maybe there is an Ethernet port speed negotiation issue with the MyQ device.  Does the MyQ documentation specify the Ethernet port speed?

I just tried that and tried both ports and still didn't light up the LED by the port for either one.  I also tried a netgear range extender and spent an hour on the phone with their it support and couldn't get that to work either.  They think that the range extender isn't compatible with the air hub. What a pain in the (Edited per community guidelines).  I might go try the inexpensive ethernet switch before I give up and return the internet air.

I should not have been on a ladder when I went through this process. Fortunately, it was only three sets of Beagle ears that were damaged in unsuccessfully attempting to reconnect the myQ app to the garage doors. I had previously purchased a basic lp-link extender (dual-band - 2.4 & 5G) to reach the Ring camera. I put the garage door openers at the tail end of the re-connection process, given the issues they presented previously. The Ring (2.4Ghz) connected easily on the resulting ATTxxx-EXT network, but not so the door openers that are in the same technology year (I am only assuming the similar specs). I've not given up hope, but it may be a long while before I waste more time on the door openers. I have no alternative to Internet Air - since its predecessor (Fixed Wireless) is going out of service next month. We live in a rural portion of GA - ten miles from fiber. FWIW - We've had Fixed Wireless for five years - it was rock solid except for two instances. Trying to find someone in AT&T to fix those issues was tough; 99% of the ATT employees thought it was a cell phone service.

With Att gateways, their band steering implementation doesn’t allow 2.4GHz only devices to connect typically on initial setup.  Temporarily disabling the 5GHz radio should do the trick.  
Using a personal WiFi system IMHO will always be better than what’s given in the gateway, but the gateway itself can be made to work with these smart home devices.