I resume this topic. after I have tried various versions following a facory reset. the problem of the active dyndns service even if disabled persists, only after restarting the router the url becomes unreachable, I presume because the ip laease has changed from the ISP.
at this point I don't think my infrastructure is the cause, the isp modem has nothing configured in dyndns.
There is no way why I can avoid rebooting the router to eject the dyndns if disabled by hand?