You need to read the documentation, and *understand* how FreeRADIUS
works.  The alternative is random changes, confusion, upset users, and
*more* work and pain.

Skipped the idea to use different LDAP attributes for WiFi and OpenVPN altogether and turned to the LDAP groups. 

But my Ldap-UserDn attribute wont get populated (even though user is successfully authenticated in LDAP). So group search obviously fails

When I manually insert my test users DN, everything works as expected. 

read the rlm_ldap manual but i still do not understand why my LDAP-UserDn is not populated. 

A.