a quote from OpenWRT Wiki:
"In the default configuration, OpenWrt bridges the wireless network to the LAN of the device. Most wireless drivers do not support bridging in client mode (see Bridged Client Mode Issues), therfore the traffic between LAN and the wireless client must be routed. The relayd package helps to implement a bridge-like behaviour with DHCP and Broadcast relaying comparable to the proprietary Broadcom WET mode."
I was able to implement the required topology using the "relayd" package following the instructions described in this link:
https://wiki.openwrt.org/doc/recipes/relayclient