StatusCode 400 "Bad request" when Connecting a Xamarin.forms app on a physical Phone to an Asp.net REST API
IIS Express cannot process requests from other machines.
You can:
Install the free extension I made called Conveyor which doesn't modify anything to do with your project or configuration https://marketplace.visualstudio.com/items?itemName=vs-publisher-1448185.ConveyorbyKeyoti
Or, modify the configuration in IIS Express How to enable external request in IIS Express?
Or, host the project on IIS proper