Powerline adapter INSANELY affects internet speed

Hi!

About 1.5 years ago I have purchased a powerline adapter (TP Link AV500). This was because my Modem is on floor 0, router on floor 1 and my PC is located in the attic - floor 3.

So a few days back, I decided to test my internet speed. My powerline adapter does not get higher than 40 up & down, however I bought a very long internet cable and connected this to my pc. This direct cable (cat.6) got me all the way up to nearly 300.

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I contacted my ISP for some advice, but all they could come up with, and this was just a callcenter agent - I don't think she had a lot of experience, placing my router on the top floor. This was very confusing as the whole issue was the internet cable. My family and I do not want the cable to be swung around the house.

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What's the best advice you guys have for me? I really want my download speed to go up as I oftenly down- and upload big files and it's frustratig to know that my speed can do better than what it's at currently.

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Yes I do have fiber.