I wrote this script that should solve your issue. Basic ideas is that this script start up just after boot and stops he NTP server on router, and waits for the time so be synced, if the time is synced, the NTP server will start, or if there is no sync in 2000 seconds, the server will start as to not run it if there is no need to sync time.
To start this script, you need to put in the /bin folder. You can upload the file to your router using WinSCP software, you can find the guide on our wiki page: https://wiki.teltonika-networks.com/view/Upload_%26_Download_Files_from_RutOS
Then you will need to execute the chmod +x /bin/ntp_script.sh, this will allow the script to be executed. After that is done you will need to edit a file called rc.local that is in /etc folder, that will start the script every time just after the device boots up. In the file, at the top, add this line: sh /bin/ntp_script.sh and then save it.
The script is in the attached file NTP_script.sh