Desktop mode can be enabled without having a remote display connected. I have tried it with two different Dell docking stations (e.g. WD19TBS) and it worked with both: Phone display was duplicated to both screens connected, ethernet connection was displayed, keyboard and mouse worked, too. Unfortunately something was odd with desktop mode enabled when not being connected to the display but I cannot remember anymore what exactly it was. Therefore I turned that off again. And since you have to reboot to swap between those modes it’s not completely convenient.