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.