Are you sure about that? I just got VoLTE working on my S7E. Mine is not from the Samsung store, but my point is that they didn't ask where I got it when I chatted with tech support. They just provisioned it, then I went into the APN settings on my phone and "Reset to default", and I now have VoLTE and video calling.
My phone only came with 2 APN's out of the box (there should be 4 total), but when I reset to default, the other 2 appeared and VoLTE worked.
Check the APN's in your phone. You should have IMS, Netsvcs, Rogers LTE and LTE Tethering.
If you don't have IMS and Netsvcs, you won't get VoLTE or Video Calling.
I do agree with you but again not every rep knows those issues. Please do remember that the reps that work at the Samsung store do NOT have knowledge of what carriers settings work and what not works. They do not know unless Samsung releases them a email and because they want to make a sell they won't tell you that a feature or two won't work due to it being unlocked. As i said before, even an iPhone 6S that was bought from Apple fully unlocked wouldn't work on VoLTE and only started working a month ago or so.