You join any solo game (campaign or custom, that doesnt matter) then open ''mindustry server'' and then on your solo game press ESC -> and press the button of multiplayer, then your friend in their PC/Mobile in main menu -> Join game -> and here use the LAN option if connected to same Wi-fi, if dont work or ya arent on same Wi-fi then press the button below and put the Server's IP address (the server is the guy that started in solo mode but remember make it server so it work)