Setting a static IP address on devices that you use a lot is quite beneficial, especially if you need to forward ports over WAN or LAN.
To set a static IP address for a device in AsusWRT Merlin, follow these instructions.
First, log in to your router's web interface. This can be done by entering the IP address of your router into the URL field of your browser. By default, AsusWRT Merlin use 192.168.1.1. The default username and password is admin for both, but it is strongly recommended that you change the username and password.
Navigate to LAN -> DHCP Server.
Once there, select your device from the drop-down list at the bottom and enter the desired IP address of the device. Once done, click Apply.
This ensures that the device will always be assigned the same IP address on your local network.