Hi,
i would like to know how to protect my routers from ethernet attack with external linux os reading nand ram and find password as mention on the following link
http://manio.skyboo.net/mikrotik/

thank you.

That article seems to be from the year 2012. Many security enhancements in Mikrotik since then.

Make sure you run at least the latest long tern version of ROS, have secure passwords and not allowing services to be open directly to Internet

This option has been fixed many years ago! As you can see this blog post is from 2012

June 2019 wasn't many years ago. That's when 6.45 removed old passwords stored using reversible algorithm. The mtpass tool still works great with backups from latest long-term 6.44.6, if they are not encrypted. Network booting would be more difficult, because you'd need different images for current boards. It's also possible that thing are stored differenly on disk, but if the system is older then 6.45, passwords are still there.

Then, how can they hack the code? My router run v6.44.6 (long-term). All services are disabled (telnet, api, www, winbox....) and they hack the password in few minutes.

The device was probably compromised before you did software update, the ONLY way to correct that is to do a Netinstall