Adding Wifi AP to Netgear Router running OpenWRT

I have bought a Netgear DGN3500B Router last year and installed OpenWRT on it. As there is no ethernet-cable from our main router to the room the Netgear router is in, its internal wifi is used as a client in the main routers wifi network and all the LAN ports of the router are used as a seperate network from the main router. This alllows for access to eg. a NAS only in the network from the netgear router, while everything connected to that network also gets the internet connection from the main router. This also allows gigabit speeds between the pcs and the NAS as the netgears LAN ports can work at 1000Mbit/s.
I have two problems I would like to take care of at the moment.
First of all the upper floor of the house doesn't have good wifi coverage (it only works in one room).
And also I would like to have access to the NAS from my phone.
Both of these problems would be taken care of, if I had a wifi access point on the netgear router. (The router is in the one room that has proper wifi) Unfortunately the router is not capable of using its internal wifi for both a client and a ap. Especially not since these would need to be assigned to different firewall groups in openwrt.
Now all four LAN ports of the router are in use, so I cant just connect a wifi repeater that acts as ap to the router. The router does however have a usb port.

I need some suggestions on what to do now. I dont want to spend more than 30-40€ on the solution.
Are there any usb-wifi adapters that would be recommended to use?
The usb adapter could also be used as the client in the other network and the internal one would be used as the ap.
Or would you recommend to buy a cheap ethernet switch and connect a wifi repeater to that switch to act as an ap?

Edit: I just figured out that the wifi adapter I am searching is probably going to be the client Wifi, as the router does not support 802.11ac, so it would be the best idea to get a ac capable usb wifi adapter and use that as the client in the houses main wifi network and have the internal wifi of the router take over the role of AP.

So I am now searching for a usb wifi adapter capable of 802.11ac, that I can use as a client in another wifi network to supply the netgear router with a internet connection.