Router: use LAN port as WAN port

Solution 1:

It is unclear that this will work although it is possible-probable even that a skilled expert could make it work.

I cant speak to this specific router but it if the device can run openwrt it can very likely be configured to convert a lan port into a WAN port, and likely by way ofsetting the ports up to connect to the chip using different vlans on the chip -> untagged ports on the "switch" side if the port. (I have done this on openwrt - albeit many years ago)

That said its high risk and not something I would consider worthwhile. Ethernet routers are dirt cheap, and (last I looked) openwrt was not user friendly - unless you happen to be a linux system admin by trade - which I am - Even then dd-wrt (which is unlikely to work on your dsl device) is way easier as it has a GUI you can do most things through.

Unless you are bloody minded, more interested in what makes the router tick or your time has no value, just spend $50 or olso on an ethernet router you can flash with dd-wrt. (That you mention you are a lay user of networkware means you are moderately likely to just brick your router and end up sad and frustrated)