There’s no apn setting on iPhones with Vidéotron... it’s all managed automatically.
It’s looks like that data roaming is not working on your device...
I understand you wrote it’s enabled... but your device is acting like it’s not enabled.
Try to disable data roaming, reboot and reenable it.
If that doesn’t work, you will have to call Vidéotron to get this fixed.
Also, you should definitely upgrade to iOS 12... it’s really worth it on older devices like the iPhone 6s.