Intranet broken when Internet address in WordPress Address

Hi. I recently loaded WordPress onto a Ubuntu Server here in my home. If I have the WordPress Address (URL) set to https://servername I can reach it from my Windows desktop, but someone accessing the page from outside my network gets broken CSS. If I change the WP Address to https://www.domainname.net my friend can access the page without an error, but I have no access whatsoever. I have to go into the admin page from outside and change it back. My router/internal dns settings are working fine. In fact if I use the external IP it resolves into the internal IP and doesn’t work when the WP Address is set to the domain.

Reguardless of the setting if I use the IPv6 IP it fails, but will work from outside my network.

It doesn’t matter to internal or external which way I have the Site Address setting.

Any insights to get this fixed would be greatly appreciated.