No one knows.....
BB shutdown of their servers will greatly impact devices next year... to what extent, we can only guess.
It is likely that your LTE/4G will work fine after the shutdown of 3G..... but how your phone will react to missing 3G signals is unknown, it might search harder for those and drain the battery. But currently your PHONE uses GSM/3G for voice calls and LTE/4G for data. The question is will your PHONE be able to utilize AT&T VoLTE Service? It's not on the list of supported devices. Some claim that they have talked with ATT Reps that say it will.... but till ATT flip the switch or adds it to the list?????