How do you block a specific WAN port #?

I've got the Cisco DPC3941T gateway and my firewall is set to "custom" but I dont see any way to add a specific port # to be blocked. How do I do that?

I'm constantly getting warnings from xfinity about known malicious IP addresses trying to access my Plex media server (port 32400). They always block them which is fine because I never use Plex outside of my LAN. How can I add 32400 as a blocked port permanently on the WAN side to stop these warnings?