Refer to the openwrt wiki:
We could setup an openwrt router as a wired / wireless dumb AP, step-by-step, easy.
Remember: connect to your main router, cannot use both wired (Lan port) and WiFi client at same time.
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[WAN] port, you could add the WAN port into the "lan" interface, simple:
[Network]-->[Interfaces]-->[Devices-Tab]-->[br-lan / Configure]-->[Bridge ports] nemu, tick the "wan" also. -
WiFi Bridge Setup (relayd), in the wiki guide,
a - setup [Local IPv4 address] to the "static" LAN port IP address, Not the [wwan] IP from the main router.
b - no need to setup [Use customer DNS servers] in the Advanced Settings.
Tested on my Xiaomi-4A gigabit router, if something wrong, please correct me.