You can try manually entering the URL and port to test your app.
Computer and phone should be on same network
After running app in Mendix Studio, make sure you are in the Native section/tab and not the responsive one after you click view app
Make sure computer and phone network is same. No VPN is involved.
Try disabling your anti-virus and firewall. Once I had the same problem, and it was the anti virus blocking a port that Studio Pro uses to communicate with Make it Native. You can configure allowing that port too, but I simply tried turning off anti virus and it worked.
If using Mendix 8 make sure you use Make it Native 8 app
Please check out common issues in the docs https://docs.mendix.com/howto/mobile/common-issues