Alright, I think I have an honest lead! I can't try it myself yet 'cause I have to pick up a router, but for those with this issue, a support rep gave me this advice:

Check the settings of security functions (such as SPI) for your router.
Also, use a port that can be used for the game.

*For details, please consult with the manufacturer of your router.

▼TCP
54992～54994,55006～55007,55021～55040
Set port forwarding on those ports.
2. If your modem/router has a firewall, disable it.

Gonna try it myself in the next day or two. Fingers crossed!