Hi @Bietitauff70
Please clarify which booster you tried. (i.E. Swisscom WLAN Box 2)
The easiest solution is the Swisscom WLAN Box (⅔, 2 is for most applications sufficient). If possible, connect it to the Internetbox by cable. This way, you get the most speed and reliability out of it, as it will automatically run in AP Mode.
While it is possible to use it as a classical repeater, I would not recommend it because it will also have the same downsides (less speed & reliability) as other repeaters-
Best,
r00t