After some reading I found a solution to my issue.

So, if anyone have this trouble :

1. Check if your firewall doesn't blocks ADB incoming connection
2. Call 10.0.2.2:(backend port) instead of localhost or 127.0.0.1 on your frontend. http://10.0.2.2:8000/api/register
3. Be sure to run expo with --tunnel npx expo start --tunnel because your machine and emulator need to be on the same network.

And now it's working.

Others Stackoverflow solutions (that doesn't work for me and multiple devs on the responses) advise to change emulator proxy with 10.0.2.2 or with the machine IP Address.