...
The traffic is probably blocked in by a firewall somewhere.
In order to have a reference check for our DSMs, I added ntp.colostate.edu as a chrony server for gully, which works, which means our router is not blocking the traffic.
chronyc sources on gully with the above server configuration looks like so, which indicates that our DSMs (with PPS from a GPS) have better clocks than ntp.colostate.edu:
...