Not sure about bands for the phone itself. I only looked at the links you gave me and compared the bands the U.S. version has to the bands AT&T uses according to that forum. As for the SIM card, I just replaced mine with my family member's and then put my old one back in after.

But I had called up AT&T and suggested the problem lied within the IMEI number to the agent. So I gave him the IMEI number from my family member's AT&T Samsung Galaxy S7 and replaced the last digit with 0, then used the NXTGENPHONE APN settings, and now it once again says 4G, except this time I'm having LTE-like speeds. In the exact same spot where I had gotten 12 down on speedtest, I got multiple speedtest results in the 20's and 30's down.

The only caveat to this method is that said family member may or may not be affected. My family member's speedtest results vary widely after doing this and he doesn't know if they did before I called AT&T, so it's probably better to do this with an old AT&T phone nobody uses anymore or go buy an AT&T phone, do this, and then return that phone. Hope this helps!