Functions:
* Modem: converts between DSL phone signal and Ethernet signal. (There have been also analog phone, ISDN phone, and TV-cable modems.)
* AP/WiFi/WLAN: wireless Access Point converts between wired and wireless Ethernet signal.
* Switch: just passes on Ethernet traffic from one port to (an)other port(s). Switch is a bridge.
* Router: acts as gateway between separate Ethernet subnets (aka broadcast domains)
* NAT: network address translation, aka masquerade (and port forwarding). Hides true addresses. On router.
* DHCP: dynamic host configuration. On router.
It seems that the Speedport has DSL-modem, router, AP, and a 4-port switch. The router probably does NAT and DHCP.
The AP and switch are the "home LAN" and the router provides addresses for all clients with DHCP.
One connects either the modem or the "WAN-port" to ISP.
The router is between the ISP and the home LAN. It hides members of home LAN from the ISP with the NAT.
The Asus has router, AP, and a 4-port switch. The router does NAT and DHCP.
The AP and switch are the "home LAN" and the router provides addresses for all clients with DHCP.
One connects the WAN-port to ISP.
The router is between the ISP and the home LAN. It hides members of home LAN from the ISP with the NAT.
Case A:
* ISP uses DSL and Speedport is connected to it. Lets call "home LAN" of Speedport "LAN-S".
* Asus WAN-port is connected to switch port on Speedport. Asus "home LAN" is "LAN-A".
* For Asus the LAN-S is the "ISP". Speedport is the ISP.
* Asus does NAT and Speedport does again NAT. Your clients connecting to Asus are behind "double NAT".
* Other clients of Speedport cannot see clients of Asus.
* You might disable AP on Speedport.
* Asus can do QoS, etc for its clients.
Case B:
* ISP uses DSL and Speedport is connected to it. Lets call "home LAN" of Speedport "LAN-S".
* Asus switch port is connected to switch port on Speedport. Asus "home LAN" is "LAN-S".
* Asus has no "ISP". DHCP on Asus must be disabled.
* VPN on Asus won't function. Asus does not route.
* Speedport DHCP serves all clients.
* Only Speedport does NAT.
* Asus serves only as a switch and as a AP.
Case C:
* Speedport magically disables its router. Modem is directly connected to switch (and AP).
* There is no separate "LAN-S", ISP's DHCP serves all connected devices.
* Asus WAN-port is connected to switch port on Speedport.
* Asus is the router that faces the ISP.
* Asus can do QoS, etc for its clients.