Why can't I change my MNC APN settings?

[AC Question]

Hey guys,

Recently got Straight Talk. Own a Nexus 5. Their default settings didn't allow me to use any data, but after finding some different APN settings, I enabled it.

I couldn't get MMS to send, but when I went to change the MNC under the APN setting, the APN auto deleted itself. This happened several times.

How can I change my APNs MNC to potentially allow me to send MMS?

Thank you!!