I had the same problem and fixed it by changing the APN to this, specifically the "APN Type" line:
Username: <not set>
Password: <not set>
Authentication type: <not set>
Proxy: <not set>
Port: <not set>
MMS proxy : 188.8.131.52
MMS port : 80
APN type: default,mms
Tap on Save in order to save the APN setting
Restart the phone and try browsing the Internet.
NOTE: The created APN will show up on the APNs screen. To activate the APN, tap on the radio button located in front of APN.
My issue was not having "APN type" correct. I needed to add ',mms' to it. ***DO NOT USE ANY SPACES! It doesn't work if you add a space.
The solution was sourced from this thread: https://forums.fido.ca/t5/Community-Archives/LTE-and-MMS-not-working/m-p/94034#
Welcome to the community!
Have you verified the data setting on your phone is enabled? While you do not necessarily need a data plan, the data setting on the phone needs to be enabled. If you do have a data plan, the data within a MMS does not count against any data allotments.
Have you verified your phone is using the proper APN settings? You can verify those settings here. Since the device is LTE-compatible, it should be using ltemobile.apn.
Hope this helps
Thank you for the additional information! I understand your phone is using ltemobile.apn. However, have you verified the MMS settings are included in that APN?
For some reason, some phones' ltemobile.apn do not include those settings. It appears to affect phones using the new Oreo version (see here).
Hope this helps