My internet is going way below the promised.

Hi Alexandre

Have you tried restarting your modem? It helps to confirm for sure that WiFi is indeed the problem by testing the speed with a device connected via Ethernet.
This seems tedious or unnecessary but it can be the difference between chasing down slow WiFi speeds and an actual problem with the line.

In addition to a factory reset of the device (not just a reboot or restart). I would check to see if nearby devices are causing interference. If you live in an apartment complex with many other dwellings close by, you may benefit from disabling auto channel selection in your modem admin settings, and selecting a specific channel. You can find which channel is least congested by using a smart phone and downloading a WiFi analyzer app, which will show you the channels of nearby devices.

You shouldn't need to do the same for 5ghz as they rarely go beyond the confines of your home, but it can happen in theory.

Hi Alexandre,
The WIFI speed is very strong influenced by environment, by distance and by processing power of the device on which you measure, etc. What I understand, you have on particular device on WIFI about 60% of the guaranteed speed of the line, and that is not bad.
Like Fizzy said, to really measure the line speed, you have to use an Ethernet connection to the modem (cable) AND, to be the only connection active (better to unplug the others) AND to switch modem WIFI to off. That is because of course, if you'll have more devices sucking data from modem, the flow will be divided between them, so the measurement will not be accurate.
In this conditions, using a PC connected by cable to modem you have to measure minimum 95% of your guaranteed speed (considering the PC enough powerful to not slow down the connection - an I3 processor or more will be ok). Also, to be accurate, before testing be sure that all programs are closed and Windows itself do not update something or so (usually -in WIN 10 only- the first 5-10 minutes after login it will continue to check on internet and load drivers, so wait till all activities are over. Close all active programs, if any. (If want to be sure, check the activity with Mask Manager -> "Performance" tab, look at CPU, Disk and Network activity they should not be "0" but almost. Close Task Manager). Open your browser, and test the speed. I use generally https://www.speedtest.net/ but on Videotron I use https://testvitesse.videotron.ca/index-en.html and on Bell networks I use https://support.bell.ca/internet/internet-speed-test .
Using the good test speed is important as using different provider to test you may have include some delays between providers. IMO Fizz use Videotron network, so I suggest to use videotron test.
Is important how you measure the speed because for example, I use now a laptop processor i7, moderat loaded (CPU>=30%) with google Chrome with 15 tabs opened, some network activity on half on them, Ethernet cable connected, Gigabit home Cat 6 network (1000 Mb max). See on image that my device measure ON CABLE only 25% of my line speed, and that is because we have another 20 devices linked by WIFI to the same modem and 2 computers working now, also because my computer is a little busy taking care of all stuff. But Bell can test line speed too, as you see, the line is ok but I see my device speed very slow. I assure you on wifi is a lot worse. Also with videotron test speed, I can see only 205 Mbps instead of 281 so only 20% of cable speed (because of supplementary delays introduced between Videotron and Bell servers).
Hope I could explain you why you may have seen a slower speed than promise. The wifi part is not FIZZ problem, is between modem and your device, they can't guarantee any speed on WIFI.
However, if you did test like I suggest and you see less than 90% for more than 2-3 days, surely contact Fizz to see if there is a problem on their side. To contact them, if you don't know how see here for example https://community.fizz.ca/questions/2250261-contact-fizz-chat-bubble-doesn-t-come
Hope that will help.
Cheers!

Alexandre,
Try to desactivate the band-steering and the ATF of your modem.
Open a session: ( username is cusadmin)
http://192.168.0.1/login.htm
For other technical help,
please, contact the Fizz customer service by chat.

Hi Alexandre. You have a lot different answers here. There was anyone who works?