The mms file has this boilerplate template:
[Modem Manager]
CarrierMMSC=
MMS_APN=
CarrierMMSProxy=
DefaultModemNumber=
AutoProcessOnConnection=
AutoProcessSMSWAP=
IMSI=
[Settings]
UseDeliveryReports=
TotalMaxAttachmentSize=
MaxAttachments=
AutoCreateSMIL=
ForceCAres=
The MMS_APN field should be populated with the telecommunication carrier’s specified URL for MMS support, along with every other aforementioned field.