I don't have 4g,LTE, only 3g+. Isn't this an 4g network?

Did you verify if your phone is compatible with Fizz https://fizz.ca/en/faq/compatible-devices
Did you configure APN parameters https://fizz.ca/en/faq/what-are-fizz-network-settings-apn

There is 4G/LTE available in most areas. Where are you located and what sort of phone do you have?

You need LTE Band 4. Some phone such as Xiaomi doesn't have it.

No blame to Fizz, I do have LTE on my phone. It may be because of your location or your phone (which may not support LTE). Indeed, I did notice that in some "underground" areas of Mtl my connexion is on 3G, but elsewhere it's LTE.

I have oneplus 3 with android 8 running on that. Living in montreal downtown

If you are having issues you can contact Fizz to work it out with them. LTE seems to be working fine at the moment.

It's your phone. It doesn't have LTE band 4 and therefore isn't compatible. You also don't have 3G 1700 MHz so you will be using a lot of Fizz Ext on that phone, I would wager. You should get a different phone if you want to stay with Fizz, and should not expect any compensation for failure to get the service you want using a device that can't support it.

@Jonathan C. Not sure on what you are basing your answer on, but the specs page of the Oneplus 3 says it has 4G LTE.

oneplus.com/ca_fr/3/specs

If you're sure that you're phone is compatible with Fizz network regarding band 4 then you should contact Fizz