External_url with the port number will make the Gitlab website inaccessible.(v16.9.2-ee)

I installed the GitLab EE on the Synology NAS Docker. I need to modify the “external_url” to http://192.168.3.33:20000 to make the hyperlink address in the notify mail auto-sent by the GitLab can be accessed to the correct GitLab website.

But the “external_url” with port number will make the GitLab website inaccessible. If I modify it to http://192.168.3.33, the GitLab website can be accessed.

If I want to make the hyperlink address in the notify email http://192.168.3.33:20000, have any other way except to modify the “external_url”?