I have resolved! 🥳🥳 I have created two sample watchOS apps, tried multiple times to install them from (iPhone's watch app). Restarted the watch 5+ times. Restarted iPhone several times. It does not worked. Then I have deleted both sample apps. Then restarted iPhone and watch (the one app automatically uninstalled, while second sample app still exists in the watch). Finally, got some break time and prayed to God. In the next try, Xcode detected my watch and "developer mode" appeared in the settings! I have used @tyxop @ahenqs comments. Thanks guys!