Thank you for the quick answer.
I have done that but it does not work. Maybe I explained myself wrong; I don't really have a clue in IT questions.
The main router uses IP address 192.168.2.254 and Address Pool Start IP 192.168.2.1 and Address Pool End IP 192.168.2.252.
I have set the secondary router to 192.168.2.2 with the same SSID and password, DHCP disabled. It is connected to the main router via ethernet.