Port Forwarding on RS700

Hi all!

I received my RS700 several days ago and I'm pretty happy with Wi-Fi speeds, coverage and setup. However, I'm struggling to get port forwarding to work. I have Pace 5268AC modem/router from AT&T and RS700 in DMZ Plus mode. AT&T's network is 192.168.1.0/24 and RS700's 10.0.0.0/24. I have some services running on my network and one of them is listening on port 7809 - when I navigate to 10.0.0.200:7809 in my web browser I can access the service. As soon as I enable port forwarding on RS700, I can no longer access this service either by using local IP or external IP from inside of my network (my computer's IP is 10.0.0.10). However, I can access this service via external IP from outside of my network. The interesting thing is I can telnet into port 7809 either by using internal or external IP, but I can't access this IP via the web interface. I tried enabling/disabling Armor on RS700 as well as settings in WAN Setup (Disable Port Scan and DoS Protection, IGMP Proxying, NAT Filtering (Secured, Open)) to no avail. My previous Netgear router (R8500) worked fine with the same home network config and ISP's equipment. I tried running Wireshark and all I'm seeing is TCP Retransmission and TCP Spurious Retransmission messages when port forwarding is enabled. UPNP is disabled.

Any help would be greatly appreciated