A community to help people learn, install, set up or troubleshoot their home network equipment and solutions.
A) I can guarantee you wasted money CAT8 cable for one.
B) You can't split ethernet like you can a coax TV cable for example or even a phone line. You would need a splitter on both sides that would limit both connections to 100 megabits per second and you would still need an extra port on the the router or switch.
What you need is a network switch. It would plug into an available port on the router, and more devices can plug into it.
You want a switch, not a splitter.
A splitter is for running two 100 Mbps connections over a single 4 pair cable, and it uses splitters at both ends of the cable.
Most splitters aren't even that. They connect both devices over the same 4-pairs. So it only works if one of the devices is powered off.
Return it. Buy a Cat6 cable and an unmanaged switch. Ethernet splitters are a scam, and Cat 8 is a cable for special applications in big data centres and nothing you own can take advantage of the ways it differs from regular Cat 6 unless you're Bill Gates.
Nobody actually uses Cat8, not even datacenters (we just use fiber or DAC). The wiki article says it was designed for it, but it's less reliable, has more overhead, and is more power hungry, so it never caught on.
You got conned by marketing. First of all, you need a switch and not a splitter. Maybe it's just your terminology, so share what you bought and we can help. If it actually is a splitter, especially since you say it says Cat7, it has to be some POS junk that should be returned. As for Cat8 cables, anything you can buy on Amazon that is Cat7 or Cat8 is probably garbage. You should only ever use Cat6 at home. I would replace those as well and pick up a CableMatters or Monoprice Cat6 patch cable in whatever length you need. Stay away from flat cables as well.