This was just a little missunderstanding, because there is no reason why a mobile phone using passive federation shouldn’t work when a Computer With a web browser Works.
Either you are configuring Your own ADFS or as a consultant configure ADFS for Your customer, you find yourself in the customers local network. Maybe physical, or With a Virtual private network (VPN).
Everything seems fine, but Your mobile Devices doesn’t find the ADFS IdP Sign-in Page.
And thats when it hits me, my computer isn’t realy on the outside as my mobile Device is. I am using local DNS Service With my local Connected computer or With the VPN active.
Resulting in local DNS lookup, but my phone uses external DNS lookup. The error is between Your external Domain hoster, DNS Service or Your firewall isn’t Routing Your external browsers to Your ADFS Services.