There's something particular to your setup (yours and the 1-2 others with the same issue), we need to figure out what's common between you.
Mine has the wan port connected as 192.168.0.254 to a DSL3700 router that has no problem updating its system date, connected to pool.ntp.org. The AC68U ntp client points to europe.pool.ntp.org, but, even addressing directly the ntp servers or changing them the system date does not update. This could be expected because I use 192.168.0.1 as the primary dns server and 8.8.8.8 as the secondary server. My dyndns client runs on the netgear modem/router.
My linux ntp client points to it.pool.ntp.org and the windows one to the standard windows server. Only the RTAC68U fails. In addition to that I have several dhcp manually assigned ip's and few port triggers and port forwarding rules active.
As told before I run Transmission with a firewall-start script while the swapon command and a couple transmission tweaks are included in the transmission startup script. The 1 GB disk is formatted EXT 4 and the USB case is a model stated by asus as compatible with my motherboard and, I hoped, compatible to the router too, attached to the USB 3 port.
I'm currently overclocked to 1000,800 with the cpu at 78 C, but all the tests I did were done with the clkfreq at 800,533.
Samba and ftp services are enabled, and the router acts as a wins server. All the remaining service, except the dlna server that is disabled, are left unchanged from the standard setup.
I'll try your command and, if it works, I'll use it as a temporary workaround. I'll report you the results of my test.