Make sure that your phone is updated. Go to the settings on your phone to see the version of the operation system.
Read more about the technical requirements to your phone on MitID.dk/technical-requirements
You can use danish, greenlandic or faroese passport, and your passport must be valid.
Your passport must have this symbol on the front page. It tells that your pasport has a hidden chip and can be read by the MitID app - and confirm you identity.