ZXHN F6600P is a two in one ONR. meaning it is both modem and router
AX73 is AX5400, while ZXHN F6600P is AX3000 so ur old own router is stronger.
It need to be set up in bridge mode, meaning using ZXHN F6600P as modem only and using ur own router for network.
related link: https://www.reddit.com/r/HomeNetworking/com...and_use_bridge/
Lowyat unifi thread https://forum.lowyat.net/topic/5424552/+9420
As I never tried with this ONR before, if u do not get response here can ask at the unifi thread. since ther will be more users with the same ONR