How to fix AT&T Mobile Transfer problem with Wi-Fi blocking phone to phone transfers

I recently switched cell phone carriers to AT&T so as to upgrade my aging Android smart phone to a newer model.  When at the store, the sales rep installed their Android app called AT&T Mobile Transfer (Google Playstore Link) on my old phone and on the new phone and began a transfer of data from old phone to the new phone.  Since I had a lot of data on my old phone, we cancelled the transfer, so that I could continue the transfer from my Wi-Fi network at home.

However, when I got home and connected both phones to my Wi-Fi network, and attempted the transfer again, the app running on the new (receiving) phone displayed the following error:

Connection Issue
Cannot connect. Your Wi-Fi provider
is blocking phone to phone transfers. Please choose a different
Wi-Fi and try again.

Here is a screenshot of that error:

I called the AT&T technical support.  The technical support rep said that the Wi-Fi should be configured for peer-to-peer connections.  My router is a NETGEAR Genie Nighthawk R7000, and so I first looked at the UPnP setting, and found that it was selected.  So both the technical rep and I were puzzled by why this would not work.

After I got off the phone, I dug a bit more in the configuration and discovered that there is a Guest Network page that has an option:

Allow guests to see each other and access my local network

The normal wireless network I have configured (not this so-called "Guest Network") does not have this setting.

When I enabled the Guest Network, and connected both phones to this network, but without enabling the above option, I still got the error message. But when I enabled the above option and reconnected the phones to the Guest Network the error message went away and the transfer started.

This fixes the immediate problem, but it is quite puzzling that this special option was only available on the Guest Network, and apparently there is no way to enable this on the other "networks" in this router.

Below is a annotated (side-note: using the handy Nimbus Screenshot Firefox Extension ) screenshot of the NETGEAR Setup/Guest Network page where the above option is to be found:

Router Firmware Version was: V1.0.4.18_1.1.52