Why doesn't speech to text work when using mobile data network?

[AC Question]

I can dictate a text message when connected to WiFi but when WiFi is not available and I'm connected to the mobile data network (Rogers LTE in Canada) I can no longer dictate text messages into the microphone. When I tap the microphone it appears to be listening but no text appears. I have downloaded the English language for offline speech to text but still doesn't work.