Reddit reviews RJ45 Ethernet Splitter Cable,RJ45 Y Splitter Adapter 1 to 2 Port Ethernet Switch Adapter Cable for CAT 5/CAT 6 LAN Ethernet Socket Connector Adapter Cat5 Cat6 Cable(1to2 RJ45cable)
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That doesn't look like any splitter I've ever seen and I looked it up, has a lot of reviews saying it never worked for them. https://www.amazon.com/Ethernet-Splitter-Adapter-Connector-RJ45cable/dp/B078WL7SFS
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Since everything was working fine before you added the splitter I would guess it's faulty. I'd suggesting getting an ethernet switch and not a splitter. A switch basically does the same thing as a splitter but better. Considering they are also cheap, you can get a 6 port one for about $10 that is what I would get. You plug your incoming signal into any of the ports and the outgoing to any of the other ports and the switch handles the multiple signals.