Remove any APN profiles you currently have installed, including the Net10 one.
Eject your current SIM, and insert the one for tmobile.
Go to Settings->General->Cellular->Cellular Data Network.
Eject the tmobile SIM and quickly insert your Net10 SIM.
Wait for the default values to load, which may take a few minutes.
Enter the following values:
NOTE: "tfdata" should be entered in every field labeled "APN", not just the one under the Cellular Data section.
APN: tfdata
MMSC:
http://mms-tf.net
MMS Proxy: mms3.tracfone.com:80
All other fields should be left blank or untouched.
Back out of the Cellular Data Network menu all the way to the main Settings menu.
Turn on Airplane Mode. Wait several seconds, then turn it off.
You should now be good to go. Test.