Wiki - attach file returns "Server responded with 0 code"


I know there are some already existing issues for this Topic.
We set up a local Gitlab Instance on a local ip (http) since we are unable to use https due to a lack of accessible ips.

With the Help of our internet provider we attached a public ip to our server which makes it possible to reach gitlab under http://ip:88
That works without any flaws - beside uploading a file in the wiki.
While uploading a file in the wiki i get the response code 0 and in Google Chrome (Console) i can see that it is an


But i can also see that the “POST” tries to connect to our local ip on port 80.

Did our provider mess something up or is it a configuration error from our side? Like do we need to enter the public ip to “external url” instead of the local
The local ip runs on http port 80

In Gitlab.rb there is no external url.
in the gitlab.yml there is

local ip
port 80
and no https

any help would be appreciated

Any ideas? pls