I am new to Zammad, and I am very happy the way this app works. I tried to find online if this application has an external option for auth, for example freeradius on another Linux server with all the users configured?
There are lots of options. LDAP, SAML, Google, Github, Gitlab, Facebook, LinkedIn, O365, SSO, Twitter, Weibo.
Radius isn’t one of the options, but it sounds like you don’t currently have a directory (I’m assuming, based on your just using freeradius as and example). So OpenLDAP or SAMBA could work for you.