The router front panel with LCD I began interfacing has a capacitive five button keypad. The panel I'm talking about was used by Sercomm SHG1500 routers and although there are multiple hardware versions of the LCD (some make use of FPGA circuits to drive the LCD, others use SPI interface), the keypad seems to be the same. Based on Semtech SX8646, it is controlled over I2C bus.
SX8646 requires 3.3 V power and is not compatible with 5 V bus. Besides I2C, it uses another pin to trigger an interrupt at host when a button is touched. The keypad PCB sits next to LCD PCB and on some hardware versions it plugs straight into it, while on others it uses a connection cable going to the router main board.
SX8646 keypad PCB
