There might be several reasons why your ethernet splitter is not working, but before we get into that it would be better to first understand the purpose of Ethernet splitter because only then perhaps you will know why your Ethernet splitter is not working.
Ethernet splitter is a device that allows signals of two ethernet cables to be combined into one single line. Then converting that line on the other end into two Ethernet cable for two devices like Computers, Laptops, etc., remember this only happens if the internet speed is from 10 to 100 Megabit per second. With all that said, it means that it won’t work on internet connections having bandwidth more than 100 Megabit per second (Mbps). So if your internet’s bandwidth is a Gigabit per second (Gbps), you’d be better off getting a switch. It really is a great, rather a standard device to divide an Ethernet connection into many other ports.

Why should I use an Ethernet Splitter?
As you already know that Ethernet Splitters work for bandwidth connections bearing bandwidths not higher than 100 Mbps. Suppose you only have a single wall plate that carries one ethernet cable to another room, but in that room you want two devices to have internet connections. So instead of carrying another wire to that room through another loophole, you could just use an ethernet splitter at both ends to solve the problem. Keep in mind, you need two ethernet splitters, one at the source point while the other at the output point where you also need a single line to be converted into two ethernet cables.
How does An Ethernet Splitter work?
An Unshielded Twisted Pair (UTP) cable has 8 wires, so its total makes 4 twisted pairs. And if you knew, the 10 and 100 Mbps connections can work on 2 twisted pairs. Therefore, an Ethernet splitter uses it’s 4 pins to carry one Ethernet signal, and other 4 pins to carry the signal of other Ethernet cable. Then, it puts all those singles to be carried on to a single line having 4 twisted pairs. Finally, those singles are divided into two ethernet cables. That’s how it works.