Fetch is undefined on IE11 when authenticating with cognito

I have an ember app where I log on and it authenticates me using ember-cognito and ember-simple-auth.

this.get('sessionService').authenticate('authenticator:cognito', credentials).then(() => {
         console.log('authenticated');
    }).catch(( reason ) => {

        console.log(reason);
    });

This works on every browser except ie which i get the error 'fetch is not defined'.

This appears in the catch section which means cognito is trying to use fetch.

I have been looking into this but cant find a solution for my scenario.