Unable to send SMS on 'Fizz EXT' Networks?

I am having a problem sending text messages when connected to any 'Fizz EXT' network on my Android phone. I am using an Asus Zenfone5 running Oreo OS, phone is unlocked and APN settings are correct.
When I select the 'Auto network selection', my phone is going to either 'Videotron 3G or 'Fizz 3G' network, which works for everything except I get no LTE speeds, just HSPA+. By manually changing over to 'Fizz 4G EXT', I get LTE speeds but I am not able to send SMS messages of any kind. The messages just sit there and eventually time out.
Anyone else have this problem? I'm fine using the 3G networks for now but would really like to get the LTE networks working so I can make use of the faster speeds.
Thanks!

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