in your sys/server.cfg at the end in a new line and save it. Restart server afterwards.
Or:
Open CS2D (Client)
New Game
More Settings
Find the entry mp_natholepunching and click at it (you find it in the category "Security And Protections"
Select "0 - Off / False"
Click Apply
Click Start to actually save it. You can instantly end the started server after that.