Hi, often in public or guest networks devices on the same network are not allowed to 'see' each other. Try using another accesspoint or tethering through a mobile device
Edit: As Achiel suggested, can you access the page using the smartphone browser? Maybe also the Mifi device does not allow devices to connect with each other.
For demo's I often connect my laptop and my iPhone to the iPhone of a colleague. In this situation my devices connect to the other iPhone, which provides the internet access by tethering. Scanning the QR code in the Mendix Developer app works just fine then. You really need two phones for this to work: One has internet access through 3G/4G and shares this connection using WiFi. The other phone and the laptop connect to that first phone. Hope this makes sense....
It was an internal firewall issue.