Should a customer require to setup a system to be able to be accessed from the internet, they will need to setup a Firewall rule that will let me open up a port. The customer need to set up a NAT rule for that port and static IP that device internally.
How to fix:
Under Network->Port Forward create a 'virtual server', defining the private IP and port, and the mapping to the public IP. Then an attempt to reach the public IP (on the WAN side) on that particular port, will be forwarded to your internal device.
NAT is on by default in the Residential series APs config. If the subnet config requires to be changed, it can be done under Network->LAN (change the DHCP pool range etc)