Roborock S7 „unable to join WiFi“

I just got a refurbished Roborock S7, it works fine except I can‘t use any of the App Features. When I reset the Roborock‘s WiFi, it‘s network shows up on my iPhone but I can‘t connect to it. I tried a different iPhone as well - same problem. Did a factory reset of the Roborock twice - doesn‘t help.

Any suggestions?