I don't like tutorials that tell you to do something, but don't explain what you are really doing- you changed something, but why?
This is very oversimplified explanation, so don't get mad at me if you know what's really going on. This is for the guys, who have no idea what port forwarding is.
Let's for simplicity think of your computer as a telephone. In the old days, people (programs on computers) could just call each other and ask "hey, can I speak with Joe (RobotRage)?"
Nowadays almost everybody has a router, which acts like this frontdesk guy, who allows you to call the world, but when somebody else calls and asks for Joe, he only hears "Nah, no Joe works in here". He doesn't know if any Joe works here, and he doesn't care. He only knows internal numbers of your company and allows them to call outside world.
What we will do, we'll tell our frontdesk guy, that whenever someone calls Joe, he should redirect the call to your phone (your computer), so you could pick it up.
But why other games work then?
Because they use servers. They first call the server and all the talking with other people goes through the server. Server never calls anyone, it just answers calls. Robot Rage was created in such way, the data of bot movement, attacks etc. goes directly from one player to another, so there have to be at least one player, who can answer the phone. That's why you're getting "P2P connection failed" errors- your computer is unable to call another guy's machine.
I'm assuming you're using Windows 7 or 10 and are connected to the network you will be playing in.
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. After logged in, find "Port forwarding". It often sits in "Advanced settings" or "Security", depending on your router.
10. Add new port forwarding rule. You might need to fill in different things depending on your router. These are:
11. Destination IP/Local IP (anything with IP really): your IPv4 that you wrote down in step 5.
12. External port/external range: 1626. If it forces you to input range, just type 1626-1626.
13. Internal port/internal range: as above.
14. Protocol: both (TCP and UDP).
15. Active/enabled: yes.
16. Save the settings. Your router is now configured to allow Robot Rage connections.