Thank you for your quick reply. I connected an old DDWRT router between my modem and lan and let qnap open the ports by upnp. The ports Qnap opened are:
TCP 80,443,8080,8081,1723 and
UDP 500,1194,1701,4500

When forwarding above ports on my old DDWRT router, I can access my Qnap server with my portable Android phone.

When forwarding the same ports on AR-750S, I can not access my server.

What could be the problem?

thank you