This is usually a problem with your MySQL database. Open it with HeidiSQL and them open the realmlist database. Click on the Data button in the top right corner of your window. Fill it out with the following information:
ID: 1
NAME: SERVER NAME
ADDRESS: Public Address (this can be a Public IP or Domain)
LOCALADDRESS: This can be the local IP of your Server or your Server's Hostname
SUBNET: 255.255.255.0
PORT: 8085
ICON: 1 (PVP)
FLAG: 64 (RECOMMENDED SERVER)
TIMEZONE: 2 (United States)
ALLOWEDSECURITYLEVEL: 0
POPULATION: 0
GAMEBUILD: 12340 (3.3.5a)
From there make sure that you've forwarded TCP 8085 to your server's private IP Address. Have your friends edit their realmlist.wtf file in their client Data directory and set it to either your Public IP Address or your DDSN Domain. If you fill out all of these details correctly, they should be able to connect.