Assigning a IP address..

This is not normal

What may be happening is cPanel may be trying to do a reverse DNS (PTR) lookup on the IP address, and that process may be timing out for some reason. What you can try is enabling Disable whois lookups for the nameserver IP manager in Main >> Server Configuration >> Tweak Settings, then trying again to assign an IP address to a nameserver.

Alternatively, you can edit /etc/nameserverips directly. This is the file from which Main >> Networking Setup >> Nameserver IPs gets its information.

Please note that it is not necessary to use this interface to assign nameserver IP addresses. The only thing that it does is edits /etc/nameserverips. It does not affect the functioning of BIND/named and does not make any changes to your zone files. It is completely possible to run nameservers successfully and never use the Nameserver IPs interface at all. Essentially, the page is presented for your information only, and does not affect how the nameservers actually work.