If you still have the issue, then you might want to restart your laptop and a phone and disable firewall.
This will for sure work. After you see it is working, enable the firewall back and allow network traffic between your laptop and a phone.
All those things helped me to get it working.
It sounds like a firewall problem of the PC you are trying it on. This could be hard to resolve when using a corporate laptop for example. Try going through the standard hot fixes on: https://docs.mendix.com/howto/mobile/common-issues