For now both T6 multiplayer and zombies work fine with 5 UDP ports open, being 3074, 4976, 4977, 28960, and 28961. Further testing will take place within the future.
Note for future readers: The ports in the above may not work for the other games such as T5.
Note for future readers: If you have enabled port forwards correctly and custom games do not work, please give your router 10 mins, then restart both your computer and your router.
(Not necessary to read) Within the future, I will remove all of the port forwards, then add them once again one by one, with tests being taken per port added within 1 hour duration each, as to see which ports are responsible for T6 custom games, I may perform tests on T5, although that is not a priority for me now. (Not necessary to read)
Dss0 I made the claim that zombies worked without UPnP nor port forwards, this claim is false as I've discovered.
(Explanation, reading is optional) The previous tests I mentioned before, such as the successful zombies test was taken after the unsuccessful multiplayer test, thus a duration of time had passed in between them. I suspect within this duration of time, my router made port forwarding changes I had made within the UI before the unsuccessful multiplayer test. I bring forth this suspicion because we use an older router than what our ISP provides today, and older routers take a surprising amount of time to make simple changes. I may not speak for newer routers, as I've not experienced one first hand. (Explanation, reading is optional)
