Some old Realtek switch chips featured a protocol called RRCP[1] where you could write to the hardware registers using a specific type of Ethernet frame. So I guess a CCP-designed backdoor would probably detect a specially encrypted WiFi packet and allow then internal memory of the device to be written/read over the air. The key would be hardwired into the chip, part of the random logic - so there will be no visible block to identify on visual inspection of the die.

Or more subtly they could insert (or just not fix) a bug which allows packet descriptors to be overwritten on reception of a certain malformed WiFi packet, e.g. too short or long, which makes it possible to overwrite regions of the device's memory and thus compromise it. A SDR might be required to transmit the malformed packet(s).

By the way, I wonder if modern Realtek switch chips might still support RRCP, and an undocumented EEPROM bit or strapping resistor might re-enable it?

1. https://en.wikipedia.org/wiki/Realtek_Remote_Control_Protoco...