Welcome to the community!
Have you verified that your data has been enabled? While the data within MMS (including photo and video messages) does not count against any data limits, data does need to be enabled to send or receive MMS.
What phone/device are you using? If it is an android device, have you verified that the proper APN settings are being used? You can verify those settings here. If your device is LTE-capable, it should be using ltemobile.apn. Be sure that it includes the MMSC, MMS Proxy and Port settings.
Hope this helps