If you cant join them, and the client, IP and Port is correct, they may not be properly port forwarding.
If this happens, you can't really play their server until they do something about it.
What IP did they provide? Was it Localhost? Did it start with 192.168? Did you join with a different IP than they posted? When was their server posted?
Viewing post in can't play multiplayer
1. Do not check local mode if you are both not on the same internet network or you are NOT using Radmin.
2. Default IP NEVER works for public servers. NEVER, only their own.
3. If they also set their IP as Localhost when hosting, duh of course it's not gonna work. This could also be caused by them having RADMIN turned off or they aren't port forwarding if their ip isnt 192.168.x.x or localhost.