Try to put a micro USB cable to your Bluetooth gamesir joystick G3s and put the other extremity to the OTG Android device.
After that you have to use the aTello application (unofficial) from Krag. Don't use the official application ( it recognize not your Bluetooth joystick)
Using android Pixel 2, I just now got my gamesir G4S working with the official Tello app.
It hasn't worked except for in the alternative "aTello" app or with the gamesir device remapping thing. Anyway, I broke down and got the gamesir T1d... but it didn't work either! This solved it for both.
The key tip for me was NOT PAIRING IT IN THE PHONE'S BLUETOOTH SETTINGS. The Tello App does the pairing.
1) uninstall Tello app
2) in bluetooth "forget" the gamesir-G4S (if you had it connected from within the phone's bluetooth settings)
3) reboot phone
4) install Tello app - accept permissions(for me they disappeared too quickly did step below)
5) make sure blue tooth is on -- but do not try to do anything in the bluetooth settings
6) go to settings -> "Apps & Notifications" -> Tello app accept permissions (Storage and Location)
7) Power-on the gamesir
8) open Tello app -> Settings -> Bluetooth controller - click on the the gamesir
go to main screen and the on-screen joysticks will be gone.
Anway, you don't need to turn the Tello itself on to get the paring done and see that it works.
After all that initial paring ordeal, it works super quickly - I can go to the Tello App's "bluetooth controller setup" screen, turn on the gamesir, and it shows up in seconds. Still have to tap it to get it connected, but this just takes a second.