Here is a link explaining NAT types:
https://www.linksys.com/in/support-article?articleNum=136980
And here is a link regarding port forwarding:
https://www.noip.com/support/knowledgebase/general-port-forwarding-guide/
What make and model router (or modem/router) do you have?
Go online to the manufacturer's website and find the applicable User Guide/Manual.
Use the manual to double check your router's configuration and run troubleshooters if necessary.
And just how much lag are you seeing? There can be other causes - just online traffic perhaps. Or an ISP throttling service.
Overall, if you are able to play, most likely the required ports are already set up. But you do need to take a look and verify that that is indeed the case.