@LaTor 

Hi,

Could you check the markings that are printed onto the Ethernet cable that you use to connect the TellStick to the router?

Normally the type of Ethernet cable used to connect devices to the router shouldn't matter with regards to the Wi-Fi, but since you are having problems and if it turned out your Ethernet cable is marked as "UTP" (unshielded twisted pairs), then it might be worthwhile to replace that Ethernet cable with one that is shielded, best would be either "SFTP" or "FFTP".

@woozle 

Thank you for quick answer. I will check when I get back to the cabin.

I used the cable that followed the TellStick unit. In any case, are you saying that with correct cable, connecting TellStick to the TP-Link router should not reduce wifi signals?

The TellStick provider (https://telldus.com/) told me that their product do not use bandwidth at all, and should not at all reduce wifi signals. 

@LaTor 

Normally, connecting a device to the router via Ethernet cable should not reduce the Wi-Fi signals, no matter if the Ethernet cable is shielded or not.

So what you are seeing is clearly something abnormal. My guess would be that something generates electrical noise that interferes with your Wi-Fi signals.

From what I see the things that can easily be replaced are the Ethernet cable and the USB power adapter of the Tellstick. Therefore I would start my troubleshooting with these if I was facing this problem myself.