Welcome to the Community
You can set a data limit on your Lg G3 by following these steps through your Settings: Apps -> Settings - > Mobile data -> Limit mobile data usage -> press OK after reading the warning -> Change cycle ( make sure that the billing cycle matches your Fido cycle, you can find it on your bill ) - >Go to the date of your billing cycle - > Set -> Limit -> Select the limit you want to put - > Set -> Warning - > Select the warning -> Set and that's it!
Also, keep in mind that your device doesn't calculate data the same way, make sure to always keep an eye on your usage through the Fido App or fido.ca .
Hope this helps!
***Edited to change title***
As they say "there's an app for that". Data manager in the google play store is a way more feature laden alternative then the built in manager. It's a free app that works in the background keeping an eye on data usage and will give you updates if requested and even turn off data once the limit is reached if need be.