This is hardly unlikely to happen, but if there is no Android Device Manager in Google Settings of your device, You need to do as follows. Finding lost Android device is only possible via Android Device Manager menu.
1. Open a browser on your Android device and search for: Google Play Services
2. On the search results, tap the Free button
3. On the opened menu, tap Play Store
4. Now, you are redirected to Play Store and you need to tap Update
5. When update is finished, go back to Google Settngs and now you should see Android Device Manager