Router accessable by dns name but not by ip

I'm about to replace my HAProxy with Traefik but I have some questions/issues by getting it to work the way I expect it.
I have Traefik up and running as docker container and set up my first router, middleware & service combo.
I can access the router by calling it with a dns name from the browser but when I try to call the router by entering the ip adress of the dns entry it does not work !?
Is Traefik designed to work only with dns names ? Shouldn't it work with IP adresses, too ? Or do I miss something within my configuration ?

The rule of the router looks like this: rule = "PathPrefix(/app)"

It should work fine with an IP Address. Caveat is that a tls enabled router requires servername in the TLS handshake.

http://some.ip.addr.ess/app should work.
https://some.ip.addr.ess/app would likely not work.