@DanTheMan827 

You could try setting the IP address on the device itself which the router should be able to communicate with if the information is correct.

Alternatively, you could also try setting the subnet to 255.255.255.0 and gives out IPs in the same range.

@Tony I prefer to have the router assign the IPs rather than configuring it on all the devices, it makes things a lot easier to manage.

My IP subnet is 10.0.0.0/8 with DHCP serving from 10.0.0.100-254

I like to put my smart plugs and whatnot in the 10.1.0.x IP range and I've actually discovered that if I set the DHCP pool to something larger that I can assign the IP and when I set the DHCP pool back to normal the assigned IP still functions.

So it appears that the Deco app is limiting IP manual assignments to the DHCP range when it doesn't need to instead of limiting it to the range of the subnet.