The game’s networking layer was specifically rewritten in Patch 1.30 to fix the very issue described here. The original game from 2002-2017 would connect to Battlenet only as a game list, and when people joined your computer then your computer acted as a true host server at the networking layer.
Then many people like you found this to be troublesome and bad, because it puts the job on the players to control their internet ports at that base internet layer. So the developers at Classic Games spent a really long time for Patch 1.30 rewriting the engine game internal system so that when you “host a game” now what actually happens is your computer tells a Blizzard server computer to host the game. So all of the games on the list are hosted by Blizzard server computers now, and only the Blizzard Entertainment company has to open their ports. This process took them a very long time and they did it specifically because they do care. However, during the process of rewriting and rewiring all of the fundamentals of how the Warcraft III game is played over a network, they made a few mistakes. These mistakes manifest themselves as uninformative “You were disconnected!” messages, unfortunately. So its up to anyone who cares about the game to find and research any specific instances of bugs in their rewritten networking layer and to report them.
So when you are trying to join Custom Games on Patch 1.30, your open ports do not matter. If you cannot join a game, then it is probably due to a programming error on the part of the Blizzard engineer who did the rewriting of the network stack. In such a case, there is nothing you can do other than try to continue to research what is wrong and then submit a technical description of the problem.
However, joining games has worked for me after the rewrite. Since there are no more server ports involved on your side, and I am able to join the game, then the most likely difference is probably your internet provider and country of residence. I have a friend who I met online who said that around the time Patch 1.30 was released, he was only able to open the Warcraft 3 game client while connected to a VPN that would use a United States location. Have you attempted this fix? Even though my friend told me the VPN is no longer necessary for starting the game, it might affect which servers your computer is permitted to contact. You should also check your outgoing firewall settings. Keep in mind that you do not need to open ports anymore. Instead, you need to be able to make an outgoing connection, the same as what your computer does when you download a webpage. Applications on your computer other than Chrome and Firefox might not be allowed to download information from the web, or something.