as far as GPe is concerned, there's no need to do any extra work to get cellular connectivity working, unless you're using sprint or one of its MVNOs.
i don't use sprint, so not sure what the exact procedure is.
if you're not sure whether your carrier uses cdma or gsm, flash either build, then go to:
settings > about phone > status
and check service state. make sure everything is in service. if it isn't, flash the other build.
the same applies to lra/acg.
1. You must unlock GSM (on stock XT1056 ROM) after every changing Baseband
2. Before flashing GPE you must delete from XML file everything about MODEM erasing
3. It`ll be better if you wipe everything on TWRP before RSD-flashing GPE