Welcome to the community!
How old is your SIM card? If you obtained your SIM card recently (ie within last couple of years), it should already be LTE (4G) enabled. If you have an older SIM card, you may need to replace it with a newer SIM card. If you need a new SIM card, you can get one from a Fido store. If you are unable to go to a store, you might consider contacting customer service so they can send you one. Alternatively, you might consider sending @FidoSolutions a PM. Once they verify some information, they will also be able to access your account. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
To which network your phone connects would be determined by a number of factors. What phone are you using? Did you purchase it from Fido? You mentioned 4G. Most phones from North America would refer to that network as LTE. If it uses 4G to denote that network, is it possible your phones are Global versions? If your phones weren't purchased from Fido, have you verified that they have compatible bands or frequencies for the LTE network? You can verify the compatible bands/frequencies here.
Have you also verified that the network selection on your phones is set to include the LTE network?
Even if your phone has some compatible LTE bands and frequencies, you should note that coverage also depends on corresponding bands/frequencies being used at your surrounding cellular towers. Not all cellular towers utilise all of the bands or frequencies. You can get an idea of your local cellular towers here.
Hope this helps 😀