Slow wifi connection for no apparent reason

Hi,

I have been having problems with my wifi connection around my flat.
My router, a Modem VDSL Comtrend VR-3026e, is plugged in a wall at about 3 meters distance from my laptop, with no physical obstructions. I am paying for a 40mb/s connection. When my laptop is connected directly to my router with a cable, I get around 38mb/s (which is normal). But once it is unplugged, I get around 2mb/s (using ookla speedtest).
My laptop is using Windows 8.1 with all updates. I have tried updating my network driver and it is up to date as well. I also updated the router software by going on my ISP's website and uploading the proper firmware using proper guidelines.

How can I fix this problem?

If you need more information please don't hesitate to ask. Thank you!

Sincerely,

Nick