The repeater amplifies the signal, for which it needs power from any source independent of the circuit, not to be confused with a powerline adapter (or whatever else they are called) that passes on the signal via the circuit. Think of it as an additional antenna that picks up a wifi signal and amplifies it from there. The fact that your internet is jerky can also have something to do with your subscription (cheaper subscription = slower internet). however, nowadays all offers from swisscom are actually fast enough for normal use, so why don't you check the reception (you can see the filled lines in the wifi sign at the bottom right of the task list)? if only one of the three bars is white and the other two are grey, this indicates that your signal is too weak, in which case you should be helped with this device). With the Tp Link you would also have the possibility to receive a wifi signal (usually a stronger antenna than the one from the laptop) and to plug it into your laptop via cable.
I hope you can do something with it :)

Thanks for the support - I have now bought the device, the increase in speed is noticeable. In fact, I mistook the thing for a PowerLine adapter.

What does the problem or solution have to do with the power? The data between the TP-Link RE550 and the Swisscom router is transmitted via WLAN.
https://www.tp-link.com/ch...

I have not yet tried it without electricity
But it works great with electricity from the socket. We also have reception with it in the garden