port forwarding issues

In the Xfinity app, I've allowed a specific port to be open on my spare PC. I've tried it with TCP/UDP and TCP only.  In Windows Firewall, the port is allowed.  If I use a website to check to see if the port is open, it is not. I've disabled my firewall to rule out that issue and when the firewall is disabled, it's still not open. I've tried a range of ports too. So, that tells me it's my router/internet. I've also set up a DMZ for that computer but no luck. I've successfully forwarded ports before but in this case it's just not working. Is there someplace else where I need to enable the port? Perhaps there's a list of ports that the router won't let through regardless of the settings in Port Forwarding section of the XFinity app? Does it take a while before the port opens or should it be pretty much right away?