If you are using PTCL as your ISP with ZXDSL831CII Modem and need to forward your ports there here is a complete tutorial with HD screenshot that will surely help you and its personally tested. If you still have any quires, feel free to ask us.
Follow The Following Steps:
Step 1: First Open Your Browser.
Step 2: Type 192.168.1.1 In Address Bar.
Step 3: Type "admin" In UserName And "admin" In Password.
Step 4: Then Click 'Advance > Lan > DHCP Server' And Click "View Reserved IP List" Button. A POPUP Will Appear Then Click "Add" Button To Add A New Entry.

Step 5: Add Your Desired IP Address And Your Server(PC) MAC Address.
Step 6: Now Go To 'NAT > Virtual Server' And Click "Add" Button Here And Add Your Desired Data Here.

Step 7: First Add A Customized Name To Your This Entry, Then Add Your Protocol, Then Add Your External Port That You Want To Forward And Then The Internal Port Where You Want To Forward External Port. Add Internal IP Where You Have Internal Port.
Step 8: Now Go To 'Firewall > IP Filtering' And Click "Add" In "Inbound traffic".

Step 9: Add Your Protocol Then Source IP(Select All To Add) And Add Your Destination IP(Internal Server-PC IP) With DNS, Add Your Internal Server-PC Port In 'From' And 'To' And Done.