DDNS is not required, I just didn't know if you were accessing the loopback through the WAN IP or through a DDNS hostname.  In the past some people reporting problems with the NAT loopback happened to be using a DDNS that wasn't up-to-date.
Check the nat PREROUTING chain for the presence for this rule:
	
	
	
		Code:
	
	
		221K   27M VSERVER    all  --  *      *       0.0.0.0/0            1.2.3.4
	 
 
1.2.3.4 should be your WAN IP
Then check the nat VSERVER chain to ensure your port forwards are all properly defined there.
	
	
	
		Code:
	
	
		iptables -t nat -L VSERVER -vn
	 
 
If you use any script that customizes the firewall (like Skynet), make sure you're not conflicting with the existing iptables rules created by the firmware.