Thank you for the support and tips.
All you were right: one extension was leaking the real IP by WebRTC. For my surprise was "Privacy Badger" of EFF.
This extension set "Use default public network interfaces only" as default. In this option de real IP of ISP leaks.
For curiosity I also tried "Use default public and private network interfaces only" and "Use any suitable network interface (recommended)". Both leaked my ISP's IP and local router IP.
Thank you a lot!