@Carl thank you.

My current setup is as follows:

Cable modem (bridge mode) >>> Main Deco (router) >>> Netgear switch >>> clients via ethernet (including slave deco unit)

Do I plug the first powerline adapter into the Netgear switch? I do not have any more free ports on the main Deco unit, as one port is connected to the cable modem, the second port to the switch.

@Carl  just FYI, avoid using circuits with GFCI outlets. Some will work okay, many will not, especially some of the CEI GFCI outlets. Older rheostats or dimmers can also wreak havoc. I recently upgraded to RA outlets and switches from Lutron after rewiring half the house and adding a few circuits to better group items and to reduce loads on a few circuits, i.e. kitchen, garage; places where tools or equipment with high LRA or starting amps get used. Some shoddy electrical work in homes built in the last 15-20 years with poorly strung & marginally sized gauge wire. I found at lest 3 pinch point where the butcher wire puller used copper staples, fired staples at that, that had pinched and broken Romex casing and as wire ages, jacket and shielding gets brittle. Caused a short to ground, which is what started me on my task of upgrading and properly isolating interrupted/ground fault outlets from the rest of the system. Just know that not every home is wied in a manner conducive to powerline backhaul, sometimes it's an easy fix, other times it can be a costly fix, whether that's paid in cash or in sweat in the attic. There are a lot of factors that can adversely affect powerline, stay as close to router as possible, create groupings based on circuits as much as possible & avoid circuits with speed controlers and rheostats or new A/C system that use IGBT variable frequency speed controls for compressor or motors. 

For the latter, they do make common mode chokes that can help absorb that "stray" high frequency "noise" (aka frequency/amperage spikes &/or stray voltage). The chokes are often used on devices prone to sensitivity, you have probably seen them on PC monitor cables and thought they were weights or just never gave them any thought, but that's what those plastic covered bulges usually are.