That should work just fine externally, assuming you are properly passing the token to it. For example this exact link works just fine on my server (before anonymizing): https://my.external.domain:32400/library/sections/1?X-Plex-Token=xNsOrTpAaTuOtKwEpNja
Although if you are developing an application and not just testing you should be sending the token as a header and not a URL parameter.
Oh!! it was how I read or interpreted how to obtain the Plex Token, i thought it was something else in the guide but i read it wrong. For some reason i thought it was the UUID, and did not pay attention to the answer staring at my face that its not that its the part at the end of the URL where you view the XML but not the XML it self. Hope I am making sense but the bottom line is i did not read https://support.plex.tv/hc/en-us/articles/204059436 this proprely :))