In order to change the gateway in an Unifi device via CLI, you should first look at the current routes.
route -e
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
192.168.20.0 * 255.255.255.0 U 0 0 0 eth0
Next, we should delete the default route.
route del default
Finally, we should register the new default route as the gateway. In my case the gateway is at 192.168.20.1.
route add default gw 192.168.20.1 eth0
If we check again the curent routes, here is the result:
route -e
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
default 192.168.20.1 0.0.0.0 UG 0 0 0 eth0
192.168.20.0 * 255.255.255.0 U 0 0 0 eth0