How do you activate VoLTE on an GSM unlocked Samsung Galaxy S5 originally from Verizon ?

I am using a GSM unlocked Galaxy S5 that originally came from Verizon. I am currently using it on a plan with AT&T. While on a call, my data would show 1x and I'd have absolutely no signal bars. What to do to fix it ? I'm aware of VoLTE and HD calling/Advanced Calling but I couldn't get it to work.