For me the Atmega88 (with internal 8Mhz oscillator) failed to run on 3V3 (supplied by the router), so I had to include an external power supply, with 5V.
Because of that, we need to make a voltage conversion between the
5V MCU <-> 3V3 Router serial.
This PSU also has a 12V rail for the RF remote control, so it can run without battery.
Signal injection into RF remote controller
You need to find your own remote control's data path, normally it is between the IC and the transistor (see image). Use a resistor for driving the transistor.
Burn the firmware using:
Oscillator: internal 8Mhz
Clear DIV8 fuse!
Firmware included in zip file. You are more than welcome to share your improvements on the software.