@Nellybel -- Three weeks is a long time.
Are your phone & your tablet any faster when going to some public location where the "ShawOpen/RogersOpen" free WiFi access-points are available?
Physically, in what city do you live? That may make a difference. In a large city (Vancouver, Calgary) in which community do you live (Surrey? Richmond? Port Coquitlam?).
After you contacted Shaw/Rogers Support [1-888-472-2222 or www.shaw.ca ] did they trouble-shoot? Did they remotely logon to your cable-modem, to check it out? Did they schedule a technician to come to your location, to test their wires (Hydro-pole to your home) ? Did they offer to replace the cable-modem?
Have you disconnected the power-cord from the cable-modem, waited a few seconds, then reconnected the power-cord, and let it completely restart, and then tried to see if this "restart" has improved your service?