The lamest way to do it is to use the quick-set menu, select the "Home AP Dual" template, and then in the template:
- configure a static LAN IP address of your router, that is free in your network.
- and remove the check from the DHCP server option.
This way eth1 will still be your WAN interface meaning that after finishing the configuration you have to connect the cable from your LAN to eth2, 3, 4 or 5.
"Home AP Dual" in Quickset is not only the lamest way to configure, but also the wrong one - it implies using device as a gateway, with one WAN port and LAN on others + wireless.
The closest Quickset template to what topic starter wants is "WISP AP" with Mode set to "Bridge".
The downside: only one wireless interface is configured automatically, so the other one need to be configured manually afterwards.
However pure WAP is actually a small and easy config, so I suggest writing it from scratch:
1) All ports to one bridge
2) DHCP client (or static address) to that bridge
3) Two wireless interfaces configured