As noted on this article, your compatible device must have been purchased from Fido. If you have a non-Fido device and no conflicting services, Wi-Fi Calling may work, but we can’t assure that the feature will work properly!
Without WiFi Calling enabled sending Secure Signal messages will not work. The only option left is sending a normal insecure SMS to which the message text has been input using the veneer of the "secure" Signal app.
Make sure that you are an eligible Fido customer. You must be an existing, postpaid, mobile customer.
I found out about the deal on a Redflagdeals post where some have successfully got the plan without being a Fido customer
https://forums.redflagdeals.com/fido-4gb-tablet-plan-cpo-tab...