Hi
I got this to work by adding specific routes even though I had a default management-route. Try this:
tmsh create /sys management-route MyNTPserver network x.x.x.x/255.255.255.255 gateway y.y.y.y
tmsh save /sys config
bigstart restart ntpd
Also, F5 documentation states:
"When the BIG-IP system starts, the ntpd process starts before the system loads the TMM network configuration, and the ntpd process attempts to use the management network to reach the NTP server. However, the attempt fails because the NTP server is on the TMM network."
(Reference: https://support.f5.com/kb/en-us/solutions/public/7000/000/sol7017.html)
Regards,
Sharmeelan