I gotta doozy for ya: I don't have a signal. So I go into Settings-->More...-->Mobile Networks--> and I change the "Preferred network type" to CDMA+LTE/EVDO. I get a perfect signal (can send calls, texts, 4 bars) until I check "Data enabled" to enable mobile data. The signal then drops to zero bars and the "Preferred Network Type" resets itself to "GSM Only".
imgurcom/NWwOfit is what I see before choosing CDMA+LTE.EVDO
imgurcom/5zaY3XP is after I choose to allow mobile data
imgurcom/xZFoE8G is my APN's after it switches to GSM only
Strangely, the APN's are greyed-out when CDMA+LTE/EVDO is selected for preferred network type.