Welcome to the community!
Sorry to hear you're having issues with MMS messages. What phone are you using? Have you verified that your plan includes text messaging? As far as I am aware, the Tablet plan does not include text messaging. Are you having issues sending SMS as well? Had you verified that your phone's data setting was enabled? While you do not need a plan with data in order to send/receive MMS, your phone's data setting does need to be enabled.
You might consider verifying your phone has the proper short message service centre (SMSC) number. Fido's SMSC is +1-514-993-1123 (see here). In addition, you might also consider verifying whether your phone has the proper MMS settings. You can verify those settings here.
Hope this helps 😀