Do you want to configure direct root login is allowed to a specific set of ip address, there are two way configure in your sshd_config file.

Note: The following configuration disabled direct root login and allow specific users and IP only,

open the ssh configure file /etc/ssh/sshd_config

#  vim /etc/ssh/sshd_config

The below configurations are disabled direct root login except  Match address (192.168.0.20) with users,
 
#LoginGraceTime 2m

PermitRootLogin no
AllowUsers username root

#Allow direct root login specific IP

Match address 192.168.0.20
PermitRootLogin yes
Match user root

PubkeyAuthentication no

Finally, restart your ssh services.
 
#  services sshd restart