Slow internet speed when connecting the powerline & DSL to the same wall outlet
I have bought TP-Link AV1200 powerline adapters (Pass through type) to expand my network from the ground floor to the first floor.
I have connected the first powerline adapter directly to the wall outlet, and plugged the DSL to the pass through plug of the powerline; so both the powerline and the DSL are connected to the same wall outlet, and connected them using ethernet wire.
Then I have connected the second powerline adapter directly to the wall in the first floor and connected it with my MacBook Pro using ethernet wire to check the speed.
My internet speed from the telecom company is about 20 Mbps.
However, after doing a speed test, the speed was about 3 Mbps!
So after that, I have connected the powerline in the ground floor to a different wall socket than the DSL and kept the same ethernet connection (Only 3m apart). The speed jumped to about 18 Mbps!
My question: what is the reason for the slow speed of the internet when connecting the DSL to the same wall outlet as the powerline adapter? and is there a way to connect them to the same wall outlet without slowing the internet speed?