<ifrane> can be responsive, but if its the
<ifrane>'s content is not responsive specifically if its outside of your domain then we can not really apply a styling on it.
It could also an issue specific to iPhone, I've checked your page again on an actual phone (Android) and the calendar is not cut off, it actually look better now than before because the calendar has more room.
Yes, better to use a calendar plugin, have you seen the Modern Event Calendar extension? does that fits what you need?