Hi @Andreas_A,

Is the shed you mentioned outdoor? Please note that TL-WPA4220 KIT is suitable for home environment, not outdoor environment.

There are several potential factors and troubleshooting steps you can take:
    1. Check powerline adapters' placement: Ensure that the powerline adapters are plugged directly into a wall outlet and not into power strips, surge protectors, or extension cords. 
    2. Test different electrical outlets and move them into the same room for comparison.
    3. Check powerline rates between the powerline devices via tpPLC utility/App, you could refer to this FAQ to understand the difference between powerline rate and actual internet speed: Explanation about the Powerline Rate and Actual speed of the Powerline Adapters
If the powerline rate is too low, refer to this FAQ to see if it will be better: What can I do if my powerline rate is very low?
    4. If the wired speed is normal, only wireless connection is slow, please try to change the Wi-Fi channel and channel width on the powerline extender.
    5. Check available firmware updates and ensure the powerline adapter and extender are both using the latest firmware.