We have a number of these (roughly 12) at my work so this is just for personal refrence but feel free to comment if it helps you.
Problem: Brother printers HL-2720DW series printer shows a amber toner light, despite new toner it will not clear.
Solution: First do everything a normal person would reboot, clean the head as instructed on every toner change. If this does not resolve it follow below:
1. Open front door the light will change color.
2. Turn off printer with the switch on the side.
3. While holding down the green GO button, turn printer back on using the switch on the side.
4. When all four LEDs light up release GO button. All LEDs will turn off
5. Press the GO button 2 times. The 3 LEDs (toner, drum, paper) will light up solid
6. Press the GO button 5 times
7. Paper light will be blinking
8. At this point the toner end-of-life condition has been reset. Close front door.