Basically, you've got 4 possible causes for this slow-down.
To verify your true & real speed; always check-out videotron's own speedtest tool at http://testvitesse.videotron.ca
I'd suggest approaching your problem iteratively with the following steps (from easiest to most-complex) while using the above speedtest service.
Another good read:
But you can also call videotron's tech-support
I had trouble using my 5ghz signal using ac only (dd wrt firmwarev24sp2) despite being able to connect to it the previous day. Could no longer connect to it with the settings as described in previous post. I finally got best results using setting n/a mixed 80 MHz channel width, extension channel upper/ lower.
now consistently getting 120mbps down and 20 or more mbps up.