iPhone/iPad users are unable to connect

Hello All,

We have installed SfB 2015 and as far as we can tell everything is working except that iPhone/iPad users are unable to use the mobile client because it can't connect to the server. Android and Windows Phone users are able to connect to the server and are using the App.

Could someone give me some pointers on where to start troubleshouting this? I would assume that it works for all, of for none...