Noob Doubts: wifi repeater/router

i'm writing because i need a "wifi repeater" as my dlink calls it, to take the wifi signal from the modem and spread it to a couple of rooms, but behave like a router (possibily on a different subnet) or to channel the signal through an ethernet cable to a proper router: can Openwrt help me? (if i try something like this with the dlink it stops repeating the signal and tries desperately to fetch it from the router...)
And if yes what router should i look for? (the only compatible stuff i have is an old asus which isn't supported anymore)

Are you looking for a scenario?

Or double NAT, with a STA WLAN interface in a WWAN setup?

I have no idea what the latter might be but the Routed Client seems what I need, or better the masked client is

would a raspberry be fit for this job?

As a wifi repeater or AP? Absolutely one of the worst options for a wifi AP in general due to the very low end, low performance radio system (single 1x1 radio, small antenna means limited speed and range, doesn't work well with multiple client devices). IIRC, it also does not work in simultaneous sta+AP modes.

For wifi, you want a proper AP or all-in-one wifi router device that has been designed to work as an AP.