I have a modem with an included WIFI router and LAN ports. To that modem, I attached an ASUS WIFI router and a NAS. To the ASUS router, I attached a sound receiver.
Try to get to one network, one subnet, and one DHCP server. This is so much simpler to manage in the end.
You can do this with your above setup.
Consider your Modem/Router to be your main network with the DCHP server here.
Perhaps at this point, reset your second ASUS WiFi router to factory specifications.
Now hook up a LAN port on the ASUS Router to a LAN port on your main network.
Give the ASUS Router a static IP address on the main network (that is, an IP address that is not within the DHCP range of the main router).
Turn DHCP OFF on the ASUS router and do a soft reset of the ASUS router (turn off, turn on).
Now everything is on one network (and that means all your devices are on one network). DHCP comes from the main router.
You can set up Wireless on either or both of the routers. If you set up wireless on both, use the same SSID and wireless credentials so that you can move between them.
This is a fairly normal setup and should work well for you.