Thank you qca9558 for the quick reply.
I meant to say flashing the openwrt build of Archer C7 firmware onto the TL-WDR7500 worked fine.
Does it mean everything is perfect and there is no error of WAN port being "shifted" to LAN4 port? Because in Post No.38 you mentioned "The same issue I was seeing before where the wan port is on the 4th lan port instead of the lan port.".
And your tip of changing /etc/config/network, was it applied in the Archer C7 firmware or the openwrt trunk?
So sorry to trouble you so much, I was given a WDR7500 but I cannot use it at all without an English interface...
Thank you very much.