I don't have an Android device to test, but you should be able to add IMAP account for GMX (assuming that this is gmx.com, not .net) manually by following the instructions under "I want to set up an IMAP or POP account" here:
https://support.office.com/en-us/art...5-f8e532091872
using these server settings:
https://support.gmx.com/pop-imap/imap/server.html