[iOS] Network request failed on device

I’m using RN 0.29.0 and Fetch to authenticate with a server and getting the Network request failed.

I was getting it on the Simulator but I saw there are many issues with Fetch and El Capitan iOS simulator such as this one.

Now I’m building for the device and getting the same error.

Obviously, when building for release I can’t see the error on Chrome but the code doesn’t work.

When using Chrome dev tools I see this:

TypeError: Network request failed
    at XMLHttpRequest.xhr.onerror (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:24332:8)
    at XMLHttpRequest.dispatchEvent (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:9864:15)
    at XMLHttpRequest.setReadyState (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:25778:6)
    at XMLHttpRequest.__didCompleteResponse (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:25636:6)
    at http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:25711:52
    at RCTDeviceEventEmitter.emit (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:9095:23)
    at MessageQueue.__callFunction (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:7183:23)
    at http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:7087:8
    at guard (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:7035:1)
    at MessageQueue.callFunctionReturnFlushedQueue (http://192.168.100.102.xip.io:8081/index.ios.bundle?platform=ios&dev=true&minify=false:7086:1)

This is the relevant code:

        fetch(url, {
            method: 'POST',
            body: 'username='+username+'&password='+password,
            headers: {
                'Accept': 'application/json',
                "Content-Type": "application/x-www-form-urlencoded",
            }
        }).then((response)=>{
            ...
        },(error)=>{
            console.log(error.message)
        }).catch((error)=>{
            console.log(error)
        })

I thought maybe it was some configuration problem when preparing the files for production but the docs are outdated and do not correspond with the reality.