A piece of DIY automotive window tint film trimmed to the right size works great. Someone gave me this idea a few years ago.
I am not bothered by my Asus RT-AC66U but my Motorola SB6141 cable modem has these obnoxious blue LEDs that practically light up the room. I bought some film on clearance for like $5 and only used a 1 inch by 3 inch strip. I still have 99% of it left.
It eliminates the brightness but you can still see the activity if you look at it from a short distance.
-- So put me on a highway, and show me a sign. And take it to the limit one more time...