Unable to Port Forward uncommon ports - Time Warner Internet

This question is about "Unable to Port Forward uncommon ports", with Time Warner Cable internet and apps. I've been trying to port forward ports (like 6398, for a custom program I wrote, and 9987, for a voice chat server for friends and myself, but despite opening every port on my router to the server machine and having firewall rules on that machine set for those specific ports, only common ports, like 53, 80 and 443 are accessible for incoming traffic. I spoke with a T3 tech on the phone today, and he couldn't figure out exactly why this wasn't working. My previous thoughts were that Spectrum was blocking ports, but he assured me that it's not something they can do, even at the T3 level. So, he told me to crowdsource the solution and post here. If anyone has had experience with this issue, could you please help out?



Thought I'd include the hardware I'm using, in case that becomes relevant, and because they aren't Spectrum issue.

Modem: Netgear CM-1000

Router: Netgear Nighthawk X6 - (ALL PORTS FORWARDED)

Server: Windows Server 2012 - (Relevant Ports Forwarded, 80 and 443 accessible from outside, 21, 6398, and 9987 are among those that are not)

Unable to Port Forward uncommon ports?