@jhandley67 this looks like a Comcast decision to use the extra SSID capability to support their wifi hotspots instead of guest networks. Looks like Comcast expects their customers and all of the customer's guests to be Comcast customers so that any guests would automatically be able to use the wifi hotspots. I guess the thinking must be "Why would anyone invite non-Comcast users to their home?"
So, since Comcast doesn't support guest networks, looks like their licencee's can't have that capability as well.
I'll describe how it works for me. It's close to Eddiepet's diagram option 2, but the smart home devices are fully on the legacy WiFi. The Rogers Gateway is of course at the top, and it did not need any configuration. The second router has the guest WiFi capability. Log into the 2nd router and make sure the Wifi SSID is different from the Rogers gateway one, and that it's LAN address range is also different from the Rogers LAN range. This is normally already the case, as most personal routers are 192.168x.x and Rogers uses 10.0.0.x for its Local IP network. When you plug the 2nd router's WAN port into the Rogers Gateway LAN side, the 2nd router gets an address and gateway automatically, and in turn deals out WiFi addresses to your users in the 192.168 range and routes traffic to the Rogers Gateway. This is double NATing but should not require special configuration.