Sorry to revive this dead thread, but I had the exact same experience (Canadian buying U.S. refurbished from Verizon which could do MMS or calls, but not both, though data still worked for the internet).
Here's how I got everything to play nice on my Samsung S7.
1) open up the settings menu on your phone and navigate to “mobile networks”.
2) select “access point names". From here, manually input your carriers .apn information which can be found on your carrier’s website or in a physical store from the staff members there.
3) save those changes by using the three button icon which opens the drop-down menu in the top right corner of the screen and then selecting “save changes.”
4) press back to go back to the first level of the “mobile networks” page.
5) select the option titled “network mode" and choose the option “lte / gsm / umts”. When you select this option, it will ask if you would like to continue this option manually or automatically. Choose “automatic".
Your phone will now swap between network modes on its own, allowing you to use MMS, data, and talk/sms as you would expect it to.
Hope this helps!
PS. The .apn info for Fido was incorrect on their website circa August 2018. All information regarding apn settings was correct, less the bit about the MMS proxy, which should be http://mms.fido.ca rather than the numerical code that they provide.