FYI, we do not have Guest WiFi on the XB6 because Comcast implemented a different solution for their customers: Xfinity WiFi Home Hotspots
The advantage with their approach is that guests' network traffic goes out on a different logical connection from the customer's Internet traffic, so Comcast can track it separately and guest traffic does not get counted in a customer's data usage.
It is also possible that something might be changing in Comcast-land because come December 17, 2020, Xfinity WiFi On Demand passes will no longer be available for purchase. That would mean no guest WiFi for non-Comcast customers.
I don't know why they are doing this. Perhaps the WiFi passes were not the cash cow that they had hoped it would be... or... they are just closing a loophole where Comcast customers started using free Xfinity WiFi passes when they hit their data caps... or... perhaps Comcast has buckled from all the pressure and will FINALLY allow their users (and their Cable Co customers' customers) to just simply configure a separate Guest WiFi network.