Just set it up in AP mode (Wireless AP) since this router supports AP mode. In this case, the old router runs as a switch plus wireless access point. If you do not need wireless function, then you can disable wireless function as well. In AP mode, DHCP function will automatically be disabled.
Some of the very old router does not support AP mode, but then there is a way to use it up as a switch plus wireless access point, I call it custom AP mode. You connect the LAN port (not WAN port) of the router to the main router, then disable DHCP function. This custom AP mode is useful as well in some other cases where the AP mode may interfere with the VLAN function.
Reference: one use case for custom AP mode for Singtel users with ONR and Singtel TV box, the router needs to suppport IGMP Snooping.
https://forums.hardwarezone.com.sg/...tel-tv-onr-ac86u-ax23-singtel-tv-box.6681949/