How to catch a bug when forwarding ports between routers?

Question

Computer networks

Internet is connected to router 1 (Asus RT-N66U). This router acts as a router. Further, router 2 (ZyXEL Keenetic) is connected to router 1 via LAN. The router has 1 static address! and when forwarding ports (80, 8080, 20, 21, 2021) to the server directly via LAN, everything works for 5+. If you do forwarding to the server from router 1 through router 2, then there is deafening silence.
The settings are set as follows: Router 1 has port forwarding enabled for a device on the local network with a static address (the address of Router 2 is specified). Surely there is no problem here - the scheme has already been verified by a direct connection to the server. Network Address Translation (NAT) is enabled on router 2, static addresses are set for all local devices, the DHCP server is enabled, the network address translation (NAT) rules set that when accessed from an ISP (external network) via http, ftp ports, broadcast to the designated local static server address on ports 80, 21, respectively.

Answer 1

Well, obviously the problem is somewhere on the second router.
Check all the settings carefully - does the address on wan match the one that is being forwarded from the first one?, is the forwarding configured correctly on it, do the forwarded ports match those that were forwarded from the first server?
Enable upnp on kinetics.
And most importantly - why do you need such a tricky network structure? Why arrange a double nat?
Isn't it easier to put them on the same network and not catch access bugs to the device behind double nat?

Answer 2

Check the security settings on the routers (sometimes called application layer gateway, ALG).
Check if the connection is correct (the wan port of router 2 is connected to the lan port of router 1).
Check addressing.