Copio y pego el contenido de la guía de usar openwrt como puente cliente wifi para dar red a clientes conectados por cable al puente (sólo la parte importante) por si la web original desaparece en un futuro, ya que me fue difícil de encontrar:
WRT Router Series Wireless Client Bridge Mode with OpenWRT
Assumptions
It is assumed the reader has a general knowledge of networking (IP Addresses, Wireless, etc.) and OpenWRT
To save confusion and focus on clarity of this specific subject, it is assumed the router being configured has a fresh installation of OpenWRT (19.07.05 or higher) installed and the following items are configured as described;
The default wireless configuration has been deleted
The WAN interface of the router has been disabled and is configured to not start at boot time.
Prerequisites
opkg update
opkg install relayd luci-proto-relay
Configuration Files
/etc/config/wireless
config wifi-device 'radio0'
WhatEverDefaultsArePresent
config wifi-iface 'wifinet0'
option ssid 'WhatEverSSID'
option device 'radio0'
option mode 'sta'
option key 'WhatEverKey'
option encryption 'psk-mixed' OR 'WhatEverOtherEncryption'
option network 'wwan'
/etc/config/network
WhatEverExistingWorkingSettings, "plus the following added or modified items";
config interface 'wwan'
option ipaddr 'WhatEverIPAddress'
option netmask 'WhatEverSubnetMask'
option gateway 'WhatEverGateway'
option broadcast 'WhatEverBroadcastIP'
list dns 'WhatEverDNSServer'
option proto 'static'
config interface 'ClientBridge'
option proto 'relay'
list network 'lan'
list network 'wwan'
option ipaddr 'SameIPAddressAsWWAN'
config interface 'lan'
WhatEverExistingWorkingSettings
MakeSureNoDefaultGatewayIsAssignedHere
option ipaddr 'AnIPAddressOnTheSameSubnetAsWWAN'
/etc/config/firewall
config zone
option name 'lan'
option input 'ACCEPT'
option output 'ACCEPT'
option forward 'ACCEPT'
option network 'lan wwan ClientBridge'
The above settings for /etc/config/firewall can be viewed or configured in the LuCI GUI via Network, Interfaces, "LAN" Interface, Edit Button, Firewall Settings Tab, Create / Assign firewall-zone DropDown (not Network, Firewall).
* Anything that starts with "WhatEver" in the above example should include appropriate settings. See the next sections for some examples to clarify things.