If you have your ISPadmin located on private network and wish to access it from the Internet, the easiest option for you is to set 1:1 NAT on public IP address to private address of ISPadmin. 
However, we do not recommend it since all ports and services on the server would be subject to DoS and BruteForce attacks etc.
Otherwise you have to redirect selected ports to private address of ISPadmin:
| ssh | TCP/22 (Port 22 is not recommended to be open, you’d better remapping it to an unexpected one (e.g. 24785 -> 22). |
| http | TCP/80 |
| https | TCP/443 |