Hi soapfan1969,
I understand you are not able to send MMS messages on your iPhone 5s after a recent data reset by your carrier. Is it possible that your carrier did not turn the text-messaging plan back on when the data was reset? Have you restarted your iPhone since the changes were made?
Restart your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT201559
Use Messages with your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT201287
You can use iMessage or SMS/MMS on an iPhone, iPad, or iPod touch.
To use SMS/MMS, you need a text-messaging plan. Contact your carrier for more information.

iMessages are texts, photos, or videos that you send to iOS devices and Macs over Wi-Fi or cellular-data networks. These messages appear in blue text bubbles. To turn iMessage on or off, go to Settings > Messages.

If you aren’t using iMessage, you can use SMS. SMS/MMS messages are texts and photos that you send to other cell phones or iOS devices. These messages appear in green text bubbles on your device.
If you can‘t send or receive messages on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT204065
- Make sure that you’re connected to the Internet through cellular data or Wi-Fi.
- Go to Settings > Messages. Check that the type of message that you're trying to send is turned on (iMessage, SMS, MMS, or Group messaging):

soapfan1969, give these initial troubleshooting steps a try and let us know the outcome.
Have a great day,