I'm assuming you're using Windows 7 or 10 and are connected to the network you will be playing in.
First we'll try automatic way- uPnP. It allows the game to ask router to pass through connections. 1. Open start menu and type "cmd", run the CMD program.
2. In the black window type "ipconfig" and press enter- this will get us informations about your's computer connection.
3. You will most likely get 1 or 2 connections. Ignore Bluetooth connections, any connections without IPv4, Default gateway or with IPv4 starting with 169.
4. If you still have more than one connection that works for us, just try the following steps for all of them.
5. Write down IPv4 and Default gateway of your connection, you'll need them later.
6. Open your internet browser, and in the field where you put website address, type http://your-default-gateway, eg. http://192.168.0.1
7. If the website doesn't load, try using cable instead of wifi- some routers don't allow access to control panel from wifi.
8. You will most likely get asked for login and password. Check on your router, it's usually on the sticker on the bottom. If it's not there, try googling your router's default login data. It's often admin with empty password or admin/admin. If you cannot find it, you might try calling your ISP.
9. Find the uPnP setting, and make sure it's enabled. Save the settings. Try to play now.
If that didn't work:
Follow these steps only if you still get P2P errors. We'll try manual port forwarding.
10. After logged in, find "Port forwarding". It often sits in "Advanced settings" or "Security", depending on your router.
11. Add new port forwarding rule. You might need to fill in different things depending on your router. These are:
12. Destination IP/Local IP (anything with IP really): your IPv4 that you wrote down in step 5.
13. External port/external range: 1626. If it forces you to input range, just type 1626-1626.
14. Internal port/internal range: as above.
15. Protocol: both (TCP and UDP).
16. Active/enabled: yes.
17. Save the settings.