Specify your computer's IP address, and the port number that your Web App responds to - as set up in IIS.
Use your computer's network IP address instead of 127.0.0.1. Otherwise, you won't be able to test it from an emulator or a device (where "127.0.0.1" doesn't mean your development Windows machine!). 127.0.0.1 will only work on your local UWP app version.
In Windows Firewall, add an in-bound rule to open the port number you use to enable iOS and Android to connect to your Web Api.