RBR750 Guest Network visible while guest network is disabled

Thanks for your reply.

Yes, after factory reset I started from new (configured WiFi Name, Password and so on). I'm from Germany, my ISP is "Deutsche Telekom" and the modem is "Fritz!Box 7490 with latest OS.

Our home is about 85 square meters, RBR (1st floor) and RBS1 (2nd floor) are in a distance of 3,5 meters in a straight line but through a concrete ceiling. RBS2 (0th floor) is about 7 meters away through wooden stairway.

Channels are configured in: 2,4G is on channel 1, 5G is on channel 36. After weeks of testing this was the most stable variant.

WiFi neighbours are there, I can find 5 other Networks in 2.4G area, but nothing in 5G.

My problem is not the performance, when it is working, everything is fine. My problem is, that it keeps kicking out my iPhones and iPads and Laptops (HP Elitebook as well as MacBook). Yesterday I renamed "Netgear_Guest" to something different (new name: TestWiFi), because it could be, that some other neighbour uses this. But now when my iPhone gets kicked out I can see Guest-Network "TestWifi", so it has to be my Orbi which is sending out "Guest Network" for a short time although it is set to OFF.