@TP-Link Found a firmware update 1.4.0 Build 20240718 Rel. 44385 - seems to have a fix for this and allows to set WAN port. However, since installing it the router seems to lose connection...

Absolutely horrible design and customer support on this front. All this time and still no solution to dedicate a port for WAN? I have recommended the deco devices to several people but never again....