Topology: Incoming Traffic > Modem > Router > Switch > Rest of Network In this image, notice IEAS-FILESVR, incoming traffic goes to the modem/router "IEASOFFICE" into the switch, to the FILESVR through the opened ports, but due to the double NAT conflict between the modem and router, the ports cannot be opened.
A DMZ is set up in the modems setup page, allowing the router to appear as the Modems WAN IP outside the firewall. With this enabled or disabled, the ports still remain closed. NAT Themselves are both enabled on the modem and router, as when they're disabled on either of them, the connection to the internet is lost.
The modem (ActionTec R1000H) is ISP supplied and cannot be replaced, as they do not support any non-ISP supplied modem, and the router in question is the TP-Link TL-ER604W.
In the modems "Port Forwarding" tab, the ports 443 and 80 are opened on the IP of IEAS-FILESVR, the ISP is not blocking these ports from being opened, when using an online port checking tool, port 23 is the only one that appears open, and nowhere is it being told to be opened. In the "Port Triggering" section of the TP-Link, ports 443 and 80 are triggered to open when they receive incoming traffic.
An attempt was made to solve this problem by using a PPPoE network between the modem and router, but failed. Firewalls have been disabled to see if they cause or contribute to the problem.
How can this double NAT between the modem and router be fixed? As disabling NAT on one or the other does not work.