Just to let the readers know the solution I found for my issue (now, it's working like a charm):
Connect the LAN port (LAN1 to be sure) of the gateway (gateway=modem+router, in my case a ZyXel) to the WAN port of the router (in my case ASUS RT-AC66U)
Something similar to this (hardware brands are just exemplary):
On the gateway GUI/portal:
- go to "broadband" section or similar, set it to bridge mode
- go to "home networking" section or similar, disable DHCP
- go to "security" section or similar, disable firewall
On the router GUI or portal, got to WAN section:
- clone MAC address (your ISP may block any device except the one with the MAC address of the known gateway). My router has that option:
set to automatic IP or DHCP (if your ISP does not give you a static IP, like it is normal)
in my ASUS I still set DHCP query frequency to "normal" instead of "agressive"
Now I really have a nice bridged network between both routers, that is the 1st router (embedded in the gateway) and the 2nd router, being my 2nd router the main only true router (routing data with firewall, NAT, etc.) and not being merely a repeater or an access point.
I did this because the 1st router provided by my ISP has much poorer performance than the 2nd router which I bought separately.