how to Battle online on pokemon diamond?

First, you need to obtain the friend code of the person you wish t battle, and the other person needs yours. Go to the wifi room (basement of any Pokemon Center) at a time that's good for both of you. Go to the front of the room to where there is a screen, and both of you should specify what kind of battle you want to do. You will start battling after a prompt asks if you want to battle this person.

did u say modem? u can't connect with a modem u need a router and the router security setting should be on WEP Key not a WPA and the WEP key is 13 digit/letters long

to change your router setting u open a browser and type in something like this : 192.196.1.1 if this don't work than u still open ur browser and open control panel and than click network connections and than click local area connections and than click support and than type in what you see in defualt gateway on to your broswer and it should open u a log in thing it should be admin for both or if u have it from ur router from the company that gives u internet than it should be admin and password should be the name on the company and than go to network setting on wireless and u should see all ur router setting and change it to a wep key

this should help you and give me 10 points. thx