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.