Many routers have a separate Access Point mode. Setting the router into Access Point mode allows it to be both an AP and a switch, I have checked and verified this. I don't think the switch behaviour can be disabled at all in these routers.
I can also expand a bit on @guest's point about moving the router-facing cable from the WAN port to the LAN port. Most such wifi routers effectively have a
- Routing block, connected to the WAN port on side and to one port of the internal switch on another.
- The internal switch itself which is very nearly unmanaged(unmanageable?) - it has virtually no parameters to control.
- A WiFi access point "bridged" to the internal switch via another one of the latter's ports.
- The management interface which may sit within (1) or be separate.
Any packet arriving at the internal switch is simply switched blindly. If the destination turns out to be accessible via the AP port, the AP converts the packet to 802.11 from 802.3 format and broadcasts it (and vice versa). Much of the configuration controls either (1) or (3), individually.
The reason I know this is that internet has mostly worked for me on a few occasions even when I or someone else had unknowingly plugged in the WAN (ethernet) cable into one of the LAN ports instead.
@davidgo's suggestion of keeping it in router mode but disabling WAN and DHCP is not the same as AP mode since several Layer 3 functionalities like NAT(which you may not be able to disable individually at all)/port forwarding/UPnP and firewall would still be running on the router consuming CPU cycles and RAM. So if you have an AP mode, use it.