Why it isn't possible to set a custom name to a found device
You can only set custom names to devices whose MAC address is found. The name has to be assigned to something and this something cannot be an IP address because IP addresses can change over time while MAC addresses stay the same. If the MAC address cannot be found (e.g. using the scanner over VPN or the scanned LAN segment is isolated from the connected LAN), it isn't possible to set the custom name and the custom name field shows N/A.