Why InAppBrowser in iOS phones opens Safari?

Hi,
I’m using this code to open InAppBrowser to show external sites into an Ionic4 app.

          const browserRef = this.iab
            .create(
              paymentUrl,
              '_self',
              'toolbar=yes,fullscreen=no,hidden=no,location=no,clearsessioncache=yes,clearcache=yes,zoom=no'
            );

          // Event raised when the browser loads a page
          browserRef.on('loadstop').subscribe(event => {
          ...<my_code>
         }

This code works well on Android, but on iOS it doesn’t work because the iPhone opens Safari into an external page ().
Is there a way to solve this problem?
Or does it exist another browser I can use in a Ionic4 app?
I can’t use iframe.

Thank you

cld