[Observium] adding private ip address

Dear Team,

We are trying to add private IP address to the observium. Our picture: the observium server is having a private IP address tho' NATed to a public ip address. I am trying to add the private IP( of a switch/router), and it gives me the following error

Try to add 192.168.128.1: Could not resolve 192.168.128.1. Devices skipped: 1.

Does this mean that, the devices can't be added using their IP? Or should the IP be DNS mapped( resolvable )?

Regards