Hi there @mpous ,
I think I picked the wrong lead due to the fact that I was missing the WiFi Connect diagram.
The issues is that the React (UI part of the Captive Portal) sends only the credentials to the WiFi (SSID & Password) and on the server side (RUST) the captive portal is stopped forcefully
Although I have created a reply on this thread since its more related to the actual context and hope I’d get attention of josephroberts’s kind interest to take a look and offer me an input about that 
After multiple tryouts and wrong leads I know that I only need to figure out how to avoid the rust to kill the captive portal just before getting the response from the /connect endpoint.