Your issue has nothing to do with Obihai, actually. It's just a very basic computer networking "how to" question.
Network routers for home use usually have a 5-port Ethernet switch built in. On the back of the router are usually 5 female Ethernet jacks. Ethernet cables plug into those jacks, and the other end of the cables plug into computers or other network devices, like an OBi box. I can't make any sense out of your post, with regard to where the cables are running. You mention a "splitter", and like drgeoff said, this is probably another Ethernet switch.
Router-->Switch--->computers and other gear
Find the switch. Plug another Ethernet cable into it and plug the other end of that cable into your OBi.
If this doesn't make sense, you will need to find someone who understands networking to help you; it's too hard to explain this by typing messages back and forth.