I'm using the M9 Plus by the way. Unless the Deco is seriously giving less prio to ICMP packets but yea it's not like this device is for advanced users so no documentation about that to be found... :...

@Knut-Magne Same problem here: (this was taken just now, via wifi laptop -> Deco - wifi backhail - > Deco - cat6 wired ethernet backhaul - Deco -> ISP CPE.) boy@MacBook-Pro-van-Boy ~ % traceroute...