So before investing in a "good" time source, I bought a cheap usb one and tried to use the instructions here to complete the config.
The problem is, the version of NTPD included with openWRT does not pay attention to the config file mentioned in the documentation ( /etc/ntpd.conf ). Instead it refers to /var/etc/ntpd.conf which by appearance is generated on ntpd start from /etc/config/system