well, if you have one reason that requires an IP address - that is it you have to have an ip address.
However, i suspect if you configure NTP/SNTP client in broadcast mode, you could have time set without IP configuration on the router. As all that is happening, your router is receiving UDP packets broadcasted by NTP server in broadcast mode.
In this case you should rather investigate why there are DHCP problems - if your APs don’t reliably get an address, it might happen to clients as well.
For using your MT as NTP server you need to install the NTP package. Did you?
-Chris