Ethernet Help

I really need help figuring this out. My wifi connection is fast right now because my family just upgraded and it gives me at least 300 mbps but the connection isn't very consistent.

So I plug in cat 5e ethernet cable to a nearby booster, and it makes the connection worse. I start to get 50 mbps at most with decent stability. I don't know why its being like this, can someone help?

I have already looked into changing router settings, checking the MTU for both the main router (some reason we have it in the garage) and the nearby booster. I am at a loss!

EDIT: my last booster was on my desk so i could use ethernet easily and it worked perfectly. the new booster is on my dads desk so it can reach a security camera. I have bought and tried a few mini extenders that have ethernet ports and dont need co-ax cables.

one plugs into the wall and recieves the main wifi from the garage, doesn't work fast but it doesn't support the 5g either so that might be it.

the other one has two wall plug ins that work together, one connected to the booster (the booster has two ports both used so i had to buy and use an ethernet port hub, if that matters) and one by my computer to plug in. the catch is that they MUST be on the same electrical circuit, so I can't connect to the main 5g router. I guess the nearby booster is actually only 2.4 ghz

If I really want to get this down my last choice is to buy yet another option that supports 5g and recieves the main connection coming from the garage, and i will have to return everything else. Thanks to everyone who gave suggestions so far.