First of all you will need to forward several ports in your router.
Step 1: login to your router as an administrator ( for most of routers default login is "admin" and password is "admin").

Step 2: go to "Port Forwarding section" in your router menu. Type: 27215 into the WAN port textbox,
(if it requires to type a port range in 2 textboxes, just type 27215 in both). Type the same port number in the "LAN Port" textbox. In the "LAN IP" type your local IP address (ex. 192.168.1.2)

(If you don't know your lan ip address, do the following:
Start > Run
Type cmd
Press enter
Type ipconfig
Press enter
In the "Local connections" you will see IP v4 lan address, that is your local IP).

Then, choose "TCP and UDP" in a protocol options protocol,
And put a tick on "Enable Mapping" if you have such an option. Then click "Add" and that's it.

The next port you will need to open is "3075", open it the same way as written above.

For more information on your particular router or modem model go to: http://portforward.com/

There are 3 in game netwrok oprions: Connection type A, B and Default.

A and B types are p2p (peer to peer) connection types, and it is good for players with slow upload speeds ( below 512 kbps), however it also requires all players to have open ports (27215 and 3075UDP and TCP). Players with closed ports loose connection and can't play. When you use these connection types all players connect and keep exchanging game data during the match.
Upload and download speed necessary to play on these connection types is only 128kbps (0.1 mbps).

Default connection type require more upload and download speed because your PC acts like a dedicated server, with which all other players connect. Only host should have game ports open, the rest of the players don't need to have it open to be able to connect and play. Internet speed requirements for host: for 2 players - 128 kbps, 4-5 players - 256 kbps, 6 -10 players - 368kbps -512kbps, 10+ players - 768kbps+

To check if you set up everythinbg correctly, create match, then minimize the game window, and go to Steam=> View => Servers, in the Server Browser window change filters, choose the game; Lost Planet Extreme Condition.

Then click - Refresh Servers, and if everythinbg is set up correctly you should be able to see your server in the list.

If you did everything but server is still not working, contact your ISP ( internet service provider), and ask them to open the ports i have mentioned.