"Using" standard NTP was a a slight exaggeration but now it's installed and tested and it didn't do any change .....
when I put the local NTP Server, 192.168.254.10, as first server in the NTP Server list and then click on "Save" a SYNC IS DONE ! and the RUT is updated, once! This is done with no Internet connection.
testing a little more it wasn't so nice...... Disconnecting the local NTP and doing a reboot of the RUT, connected to Internet, RUT is unfortunately NOT synching with the Internet NTP servers anymore.
So several testing hours later I'm still stuck !