Only the main station needs a connection to the WLAN. The two additional stations need a connection to the main station. In my case, the main station is in the house and the two additional stations are in the camper van in front of the house and it works well.

The indoor station in the basement did not work for me, I could not establish a connection to the main station (on the ground floor). It is a pity that there is no possibility to connect an additional module via WLAN.