I have a RB2011UAS now upgraded to 6.18. I was at 5.xx, but couldn't never get it to work, so I upgraded. I have tried all I can think of to get this to work.
1. turned off all firewalls
2. used different and/or multiple or the same time servers
3. upgraded firmware
4. specifically open ports 123 all directions.
5. browsed the forums, couldn't find anything that worked.
6. install NTP package, same thing.
The client seems to be started, but the clock never sets. I had debug/logging running on the built-in NTP client, didn't tell me anything, mostly that it is waiting 16 minutes to not work again. Something like that. Those particular logs are gone as I had to reboot as I installed the NTP package.
I guess I'm out of ideas other than replace the box.