I have an iSY994y/2413S combo and I have successfully set up a number of other Insteon switches on this setup (2477S, 2466SW, 2334-2). However, I have a handful of 2476S and 2476D switches, and none of them will pair. Here is what I tried:
Link Management -> Start Linking in the admin console, then hold the Set button for 3 seconds. The switch never shows up in the console. The same procedure worked fine for the 2477S.
Factory reset the 2476 by holding Set for 10 seconds while applying power. It doesn't beep and I can't tell if this has an effect.
Tools -> Diagnostics -> Event Viewer -> Device Communication Events. I don't see anything in here regarding the 2476, even if I press/hold the button on it
The 2476D (dimmer) switches buzz for a few seconds right after applying power. I don't know if this is normal, but every single one of them does it so it would be odd if they all suffered from an identical hardware failure.
The 2476 switches are all able to control the load by pushing the paddle switch.
All devices are on the same AC phase and local to each other.
I don't think the 2466SW is a dual band device so if I was able to successfully pair it, that means that powerline comms are working correctly.