Router falsely blocking friends joining server

I have set up a minecraft server on my home computer. However, my friends cannot join it, since they are on other WIFI networks. My brothers computer can join it when he is on the same wifi network, using my computer's IP address. But it seems like the router is blocking my friends whenever they  try to join. My router used to let my friends join no problem, but a few months later we want to use the server again, and it blocks them. This has also happened in my Nintendo switch when they try to join a Smash Bros server. Does anyone know what is causing this, and how to fix this?