Yeah, I think the RTC is only consulted on first boot. Once the system is up it restores the date from the RTC and then keeps it in sync with NTP. If you do not have an accurate time in the RTC then and no ability to use NTP then you will need another mechanism. I have used rdate in the past to do some setup but I don’t know if it requires UDP or not. Nor do I know if it integrates properly with systemd-timesyncd.
Drew