To configure a shared connection you need to make the IPv4 connection method as “sharing”. See this manual page for more details:
https://wiki.debian.org/FreedomBox/Manual/Networks#Internet_Connection_Sharing
The mDNS auto discovery of hosts does not work on interfaces in “external” zone (default). Once you have your additional interfaces working (when you set them up as “shared” mode), freedombox.local will work on that.
BTW, all this should not have stopped your domain setup from working. Does your Wi-Fi router get a public IP address when you connect to your ISP?