My software must be different on my G3100. For me it was: Advanced > IPv4 Address Distribution (located in the Network Settings box) > Connection list > *find the connected device in the list then click Edit next to that device > tic the box next to "Static Lease Type" > Apply.
When you are returned to the list of devices, you should "Never" in the "Expired In" column next to the device you just modified.
*If your device is not already assigned an IP, scroll down to the bottom of the list of connected devices and you will see the button, "Add static connection."