I want to set up one WHR-G54S attached to the cable modem via the WAN
port. (this one, now DD-WRT)

Then I want to use another WHR-G54S as a wireless access point where
computers will be plugged into the LAN ports.

Which Wireless Mode should the cable-attached router be set to?

Which Wireless Mode should the remote router be set to?

Wireless Mode
The wireless part of your router can run in different modes:

* AP mode – This is the default mode, also called
Infrastructure mode. Your router acts as an central connection point,
which wireless clients can connect to.

* Client mode – The radio interface is used to connect the
internet-facing side of the router (i.e., the WAN) as a client to a
remote accesspoint. NAT or routing are performed between WAN and LAN,
like in "normal" gateway or router mode. Use this mode, e.g., if your
internet connection is provided by a remote accesspoint, and you want to
connect a subnet of your own to it.

* Client Bridged mode – The radio interface is used to connect
the LAN side of the router to a remote accesspoint. The LAN and the
remote AP will be in the same subnet (This is called a "bridge" between
two network segments). The WAN side of the router is unused and can be
disabled. Use this mode, e.g., to make the router act as a "WLAN
adapter" for a device connected to one of its LAN ethernet ports.

* Ad-Hoc mode – This is for peer to peer wireless connections.
Clients running in Ad-Hoc mode can connect to each other as required
without involving central access points.