Here's an example:
/server irc.undernet.org 6665
/server irc.undernet.org 6667
As a note, some servers/networks support ports that others do not support. If you check the website for a network, it may tell you the supported ports. Otherwise, you can just test with common IRC server ports.
You might also try connecting using the server's IP address instead of the name. For example:
/server 18.104.22.168 6667
Note that the IP listed there isn't a valid IRC server IP address (as far as I know). You'd need to replace it with a valid one. You can find server IP addresses on the network's website or by doing a lookup by the server name or a variety of other ways.
Regarding your comment about websites... you cannot connect to a website within mIRC. That's why you receive unable to resolve server.