After hours of my own research, nothing seemed to work. When looking under my iptables rules I noticed no packets were captured by my original rule of:
iptables -I INPUT -p tcp --dport 30000:50000 -j ACCEPT
That's the rule everyone says to use to resolve this issue, unfortunately it didn't work for me. That is when I noticed the chain called "cP-Firewall-1-INPUT". Bam! I added the rule under that chain and SUCCESS!
Solution: (If you have a chain named "cP-Firewall-1-INPUT"
Un-comment below lines in /etc/pure-ftpd.conf
PassivePortRange 30000 50000
Step 2: Allow the port range in iptables
iptables -A cP-Firewall-1-INPUT -p tcp --dport 30000:50000 -j ACCEPT
Step3: Then save your changes: