Yeah you got it. Set the listening interface to eth0 or whatever the wan interface is. Might need to get full ntpd instead of the busy box version. The Interface option here: https://www.ntp.org/documentation/4.2.8-series/miscopt/

I’m actually setting up a router with a rpi4 right now, only using Fedora IoT instead of OpenWrt. It’s a bit more assembly required lol