We have recently set up a modded 1.12.2 forge server to play modded minecraft. Both the host and 3 other people are able to play perfectly fine, but for some reason one of our friends is completely unable to join. They have the same version of forge as us, all of the mods correctly installed, and the right server ip adress, but for some reason for them it shows the server as constantly down and unable to be connected to. They can connect to other servers just fine, and to our vanilla server. We've tried everything we can but this is such a strange issue so I don't know what to do.