CDMA network is for CDMA providers (Sprint, Boost Mobile, Virgin Mobile, Verizon etc...), if you're using a GSM provider (AT&T, T-Mobile etc..) you need to set the Network Type from the phone settings menu, to do it go to:
Settings > Connections > Networks > Network Type/Mode and change it to "GSM/UMTS" or "GSM/WCDMA auto".
If your phone is rooted the APN for your new carrier will be automatically set, else you also have to manually input/set it so your new providers data/internet will work. For your convenience we have listed the APN (Access Point Names) for common GSM Providers/Carriers in a APN settings topic of our forum: https://idoneapps.com/forum/index.php?topic=23.0
About 4G what carrier are yo using now? Your phone does supports these LTE bands:https://www.frequencycheck.com/models/OemwJ/samsung-sm-g920p-galaxy-s6-td-lte-samsung-zero-f
B4 (1700/2100 AWS 1) LTE
B5 (850) LTE
B12 (700 ac) LTE
B25 (1900 +) LTE
B26 (850 +) LTE
B41 (TD 2500)
Is your carrier using one of the above?