I've bought a standard light dimmer and a dimmable led lamp. Dimming works fine. However, when I dim the lights my ADSL connection drops. The DSL light on my modem turns off and I can't connect back again unless I switch off the light or turn it all the way up. What is causing this? And how can I prevent the connection drop without moving the modem to a different room?
Follow the cord from the router back through the power strip(s) and to the actual wall socket (i.e. Half receptacle) it is plugged into. Mark that wall socket with a piece of tape or file-folder "flag". Now, unplug the power strip and plug an actual floor lamp into that exact socket, and the floor lamp needs an incandescent bulb.
Now, work the dimmer. I'll bet you "best answer" that the floor lamp now dims.
A lot of houses are built with light switches that control power receptacles instead of bothering to install ceiling lights; it makes houses cheaper to build. Later the homeowners commonly add a proper ceiling light and forget/leave the receptacle as is.
Often the receptacle is "split", where 1 socket is switched by the switch, and the other socket is always-hot. This split is very easy to do, and very easy to undo.
Now if my theory is correct, you unwittingly put a dimmer on a power socket. That is bad for so many worse reasons than this.
But the solution is easy, check the other twin socket to see if it also dims, and if it does not, just read any of the answers here on how to un-split a receptacle.
Otherwise change the wiring in the light switch so this socket's hot comes off the always-hot instead of the switched-hot. Come to think of it, maybe the last guy already did this, and you un-did it by mistake when you added the dimmer.
An alternative to changing the light dimmer would be to re-wire the telephone line between the phone and the network interface device (or as far as you could re-wire it) with Cat 6, while making sure the wire is at least a foot away from any power wires. This creates better immunity to alien noise. Could use Cat 6A, it is shielded as well.