@EinfachNurLeon 

Hi, do you mean wired PC could get full internet from the extender while your phone could only connect to router signal and it is quite weak?

Is the extended network name the same as your router? If so, it is recommended to change the extended network name to be a different one for testing, then you could connect to the new extended network to see if the internet works fine.

By the way, since the extender needs to connect the router wirelessly, the extender should be placed where the router signal is still working, you mentioned that the router signal is weak or almost no signal, in that case, the extender will receive a quite weak signal from the router as well, it is recommended to relocate the extender, for example at the half way between the router and the room where you need the WiFi.