I think I made a mistake with the command. Move the quote to the left of the -f
>mail($mymail, $subject, utf8_decode($contenido) ,$header, "-freal-email-address");
I have edited your first post (email addresses)
You need to have your script send email as a real user and not the webserver "nobody".
>mail($mymail, $subject, utf8_decode($contenido) ,$header, -f"real-email-address");
You can get to it securely if you do this:
>https://your-domain.ext/webmail
When prompted to login, use your email address and password.
If that fails because of the shared SSL certificate do this:
>https://your-TCH-servername/webmail
Again, use your email address and password to login.
One thing you can try is download the latest firmware and reconfigure it. If it continues to lose your settings replace it. New routers are not that expensive anymore.