Not sure how much help I can be until you can figure out a way to look at a logfile. As I mentioned, there is a facility to create log files (which I’ve not used myself), and it looks like you enable it by defining a couple constants in /st-device-sdk-c/stdkconfig:
Look at the /st-device-sdk-c/src/iot_log_file.c to see what it does. I don’t know of any documentation on this unfortunately. Maybe someone here can provide more insight.
What kind of mobile phone do you have? I have iphone and it first asks if you want to connect to the AP. If you can verify that the device has gone into SoftAP mode and is broadcasting it’s SSID but you never see any progress in the mobile app after initiating the onboarding through Add device, then it sounds like it’s getting hung up in the TLS handshaking between mobile and device. Are you typing in the device serial number manually in the mobile app or using a QR code?