Re: Can't join server together with friend (on same internet connection)

This is most likely a router issue, a problem in your network setup. Simply upgrading your router firmware might do the trick, otherwise the following guide might help:

UPnP - Router: 

1. Log into your modem / router. 
2. Turn OFF UPNP. 

Turn off PC UPnP Service: 

1. Press: Windows-button (the one between lAlt and lCtrl) + R, a window should pop up. 
2. Write: "services.msc" (without the ") then press enter. 
3. In the list of services, scroll down to "UPnP Device Host", click it and press "Stop service". 
4. Now do the same thing but with the service "SSDP Discovery". 
5. Exit the window and go to control panel-> Network and Internet-> Network and Sharing Centre-> Change advanced sharing settings (to the left)-> Under "network discovery" select to turn off network discovery 
6. SAVE CHANGE