When should i cancel my old internet provider?

SInce the refund would be pro-rated you can cancel the day you get the install and after you've confirmed its all up and running. .

Cancel when the service is up and running properly.
Make sure to test everything that is important for you like special websites, connection of special devices (Google home,...).

Don't make the mistake and cancel your current Internet service provider before your Fizz Internet works. Chances are your Fizz Internet will work and you will eventually cancel, but only do it when you are sure everything is working fine.
A friend of mine got a faulty modem and it took the technician 2 days before he could come back to fix it. Had he canceled his previous Internet service provider, he would've been left 2 days without stable Internet.
Cheers.