Wired and wireless connections

I'm having a lot of problems with a couple of connection issues and was hoping someone would be able to help me. My dad works from home and has a modem and router and all sorts of connections running through his office. We just recently got a new modem/Router and just finished installing it and getting all the computers in the house connected. He has a cable going from the modem into a splitter, then has the cable for my stuff go into the wall and come out by my computer. The main internet cable then goes into another splitter, which then goes into my xbox and my computer. The problem I am having is when i plug in the wireless router that we have for laptops, phones, etc. the internet connection to my computer and xbox gets disabled. What can i do?