It is unlikely that the 8 way TV antenna splitter is where the internet connection comes in. While an antenna splitter can be used to distribute TV signals to multiple devices, it is not designed to handle internet signals.

To access the internet in a house, there are several possible methods, such as cable or DSL connections that run through phone or cable lines, fiber-optic connections, or satellite connections. It's possible that the previous owner of the house had set up their internet connection in a unique way, but it's unlikely that the 8 way TV antenna splitter is the source of the internet signal, unless the owner meant that you can indeed connect your cable modem here.

Wireless modems are pretty good these days so I would recommend you get one of those from your ISP.