1st, I set the Sonic Zxyel modem to bridge mode. Then, with the ASUS router, I forwarded the ports designated by the NAS for various apps through the ASUS via a LAN static IP outside the manually-set DHCP range (192.168.1.2-200) to the NAS.
From the internet, I can see port 80. I can also see a second forwarded high-number port to the same numbered one on the NAS, but I cannot get other ports to forward as they should. E.g., an app called DSAudio should use port 5000 forwarded to that same port. It connects, but does not transmit data well. A little loads, then buffers, stalls, etc. I have tried many configs, and worked with Syno support who finally concluded that, since I could forward port 80 to 5000, Sonic must be blocking 5000. Sonic says no. I need ports other than 80 to work in order to use the NAS well.
What to do?