Check your mtaserver.conf to make sure is set to 1, and that has a valid port set (default should be 22005). If you're using an external server to host the resources, be sure to put the url in . If you are not, then I suggest leaving this blank.
If this doesn't help, check that your computer's firewall settings aren't blocking needed ports (usually 22003, 22005, and/or 22010). Your ISP might block game server data, as well (unlikely, but sometimes it happens). You can also check your router configuration, as some have built-in firewalls.
I don't really know much about MTA itself so don't think I'm able to accurately answer everything, but I do my best with what I know. I hope this helps.