Having trouble getting my verizon lg g3 to work on AT&T 4G LTE in Alabama

. Could someone please help me find the correct settings to make A Verizon LG g3 work on AT&T 4G LTE in Birmingham Alabama. I have already gotten a SIM card that is supposed to work on unlocked phone from AT&T however for some reason I cannot get 4G LTE only H and when I use speed test it only runs a maximum download of about 6.5 mb/s where is the same sim card in my htc m8 will run an average of about 25 mb/s I have tried various APN settings obviously I'm doing something wrong any suggestions will be greatly appreciated thank you in advance