I have done quite a lot of searching as well and come up with little help. I have HSPA+ and MMS working by using Cricket's pushed APN (no flashing), but no LTE and I can't figure out if I even have the proper bands available in my area to test. Following are the APN settings that give me HSPA+ and MMS:
Name: Cricket AIO
MMS proxy: proxy.aiowireless.net
MMS port: 80
APN type: default,mms,fota,supl
Everything else is blank/default. This worked on NC5 stock rooted as well as MDK running CM11.
Hopefully this will help anyone else who comes down this particular rabbit hole. If anyone has any ideas on where I could find what bands of LTE are available where on Cricket's network or if the SCH-I545 doesn't support any of those bands (I think it does) then that would give me something to go on.