Secured AP for only two devices and trasmitting power

Hi there,
i need to bring wifi in my garage/locked parking spot that is in a public space only to connect two devices (a wallbox through wifi and a nas through cable.
I will use an old Tp-Link Archer C2 V1 with Openwrt 22.03.5 (upgraded yesterday from 18.xx with some problems of luci disappearing but this is another issue).

I would like that the install takes the network access from the connection in the apartment through LAN cable but that in the garage only Nas and wallbox (and maybe a couple of phones ) can connect to the internet.

I would also like that transmitting power in the garage is so low to cover only a small room (4x5 meters).

Can someone please help me in this configuration? It's quite a long time that i don't use openwrt because of needs of a 4g router (that is the main router here, a Huawei B535-232 with stock software )

Thanks!