The DIR-845L isn't properly working when using it as an access point. The NTP-sync doesn't working because it thinks there isn't an internet connection. Please see
this topic for more details about this bug/missing feature.
Since you can't change the NTP-server (you can only select 2 domains), you can't select a local address. Since I've got multiple devices that have a NTP-server (my NAS, my other router), I don't need another device that connects to the internet to check what time it is.
Please make it possible to manually choose your NTP-server, so you can select a local IP (my NAS or router) or a server in you own country (I can only select a server from Taiwan or US, while living in Europe). It makes the bug a mentioned before a little bit less irritating and adds another great feature as well.
BTW, why not using pool.ntp.org as default?