Kareem_nasser is right. You can do it in two ways:
My modem have an ip of 192.168.1.1. I have enabled the DHCP server on the modem but the ranges the DHCP server is giving are in the 192.168.1.2XX range. I have added the mac address of the router in the modem to always give it an IP of 192.168.1.200. so you can add that address to the DMZ zone. The router is on Automatic IP.
Another way:
Disable the DHCP server on the modem, give the router a static IP that is different from the modem IP. eg. modem IP 192.168.1.1, router IP 192.168.1.200. Set the gateway and the DNS servers in the router as the modem IP 192.168.1.1. Then in the modem, add the IP of the router (192.168.1.200) in the DMZ zone.
I also enabled the DHCP server in the router. However, it is giving an different IP range e.g. 192.168.2.XX to the devices connected through WIFI to the router.
My modem have an ip of 192.168.1.1. I have enabled the DHCP server on the modem but the ranges the DHCP server is giving are in the 192.168.1.2XX range. I have added the mac address of the router in the modem to always give it an IP of 192.168.1.200. so you can add that address to the DMZ zone. The router is on Automatic IP.
Another way:
Disable the DHCP server on the modem, give the router a static IP that is different from the modem IP. eg. modem IP 192.168.1.1, router IP 192.168.1.200. Set the gateway and the DNS servers in the router as the modem IP 192.168.1.1. Then in the modem, add the IP of the router (192.168.1.200) in the DMZ zone.
I also enabled the DHCP server in the router. However, it is giving an different IP range e.g. 192.168.2.XX to the devices connected through WIFI to the router.