DIAGNOSIS You can't dim them, but you can turn the front panel indicator LEDs off entirely through the router's software. There's no physical button for it on this model, which is a common complaint.
FIX Access the router's web interface. Navigate to Advanced > System Tools > System Parameters. Look for the 'LED Settings' or 'Indicator Light' section. There should be an option to 'Turn off LED lights'. You can usually choose to turn them off on a schedule (like at night) or all the time. Check the box and save the settings. The lights should go dark immediately.
IF THAT DIDN'T WORK If the software setting doesn't stick after a reboot, or if you can't access the settings at all, a very low-tech fix is to use a small piece of opaque electrical tape over each LED. It's not elegant, but it's effective and reversible.
HEADS UP With the LEDs off, you lose the visual status cues. If your internet goes down, you won't have the blinking orange Internet light as an immediate indicator. You'll have to check your connected devices or log into the interface to diagnose connection problems.