Setting up an NTP time server for your LAN - RMerl/asuswrt-merlin.ng GitHub Wiki
To use your router as an NTP time server set the Enable local NTP server option to Yes in Administration - System > Basic Config.
To intercept NTP connections from LAN clients and redirect them to the router's NTP server enable Intercept NTP client requests. Note that IPv6 is not supported.
3rd Party Addons
There is an addon package for Asuswrt-Merlin Firmware called ntpMerlin. ntpMerlin implements an NTP time server for AsusWRT Merlin with charts for daily, weekly and monthly summaries of performance. A choice between ntpd and chrony is available.