I was also having the issue where my watch was not showing up in the list of devices, nor did I have an option to enable developer mode. I followed @Igor-ribeiro-dev's advice, but with some modifications:
Unplug the iPhone from Mac
Unpair the iPhone from Xcode
**On iPhone, clear all trusted computers (Settings > Developer > Clear Trusted Computers)
Disable developer mode on iPhone
Enable developer mode and restart the iPhone
**Quit out of Xcode
Plug in iPhone via USB cable, and Trust computer
**Open Xcode, and try to run app on iPhone, at which point iPhone will prompt you to trust computer again, so do it.
Xcode should also detect your watch
Enable developer mode on watch
Run app on the watch.
I think the issue I was having was when I was plugging my iPhone into my Mac, I also had Xcode open, and so both "trust" messages weren't getting through, because I didn't have time to respond to the first one (entering my PIN) before the second one would pop up. By waiting to open Xcode until after I trusted the computer initially, things seemed to work fine for me. Hope that's helpful for anyone else that might be struggling with this.