Questions about Value RJ45 T-adapter

I only have one (output) cable for the internet at the fuse box by the inter-distribution. If I plug the cable zbw into the distribution -> living room, I only have Internet from the socket there. I would like to have internet from the socket in the office as well. So the question is, is this splitter the right one? Unfortunately there is no Lan channel with 2 connection heads.

Would this work with a devolo to turn 2 ports into 3?

So I bought this adapter, but only one ''side'' works. Apparently I'm not alone in this, if you look at the reviews and other questions. Once and for all, I would like to ask: What do I have to do (or buy additionally) so that both ports work? I would be forever grateful to the person who could also link me to the article (so that I am sure to buy the right one) ;)

Hello together I am not at all familiar with this topic and therefore my possibly somewhat "stupid" question: If I have two LAN plugs in my room and now insert this adapter into one of the two, will my general internet performance go down? Or will only the performance of the adapter be halved, so to speak? Thanks for the clarification

Unfortunately, it didn't work for me. No idea why. Unfortunately, only one of two connections works. Reconnecting is not the idea. Only one device works at a time. I have a network socket with one connection in the basement and wanted to connect two devices, but only one connection. Any ideas? House with network distribution cabinet and switch. A 2-socket on each floor except in the basement. Hence the splitter.

Does this also work via dlan via a socket?