I am confused. What Webserver are you using? nginx or IIS (Microsoft) ??
if nginx:
The delivered configurations (shipped with zammad) are totally fine and should work. You might need to edit the servername if you don’t want to reach it just via localhost.
as for IIS:
There’s -as far as I’m aware- no native Proxy-Support by IIS (you may correct me here) and thus I have no clue what might be wrong. Please note that IIS is far out of ours skope here.