Settings > Mobile data > Access point names > Menu > New APN
Enter the following:
Name: straight talk
APN: tfdata
Proxy: Not Set
Port: Not Set
Username: Not Set
Password: Not Set
Server: Not Set
MMSC: http://mms-tf.net
MMS Proxy: mms3.tracfone.com
MMS Port: 80
---- OR
Name: straight talk
APN: wap.tracfone
Proxy: Leave blank
Port: 8080
Username: Not Set
Password: Not Set
Server: Not Set
MMSC: http://mms.tracfone.com
MMS Proxy: Leave blank
MMS Port: Leave blank
Save your new APN then restart your phone.