LTE and LTE+ doesn't work on my unlocked Samsung Galaxy S8 SM-G950U1

Bought two unlocked Samsung Galaxy S8 from Best Buy, both have model number SM-950U1, one is black, one is blue. The black one works fine, but the blue one doesn't get LTE or LTE+, it only gets HSPA or HSPA+. The phones are literally right beside each other and LTE works for one but not the other. They're both activated through AT&T.

The weird part is that LTE on the blue one DID work before I called AT&T and had them activate it. But now it won't work. Can it be because they did it wrong somehow? Didn't erase a previous phone on the account or something? I have had a similar issue before and that what was wrong then.

I've done a factory reset on the blue one, still doesn't work. Tried different APN packages and settings, nothing has worked.

Any advise? Return the phone?

Update: Tried popping the other sim into the blue one and got LTE+ right away, so it's definitely on AT&T's side of things. Customer service suggested I'll get a new sim that's "LTE compatible" - which is strange because I had LTE+ on my previous phone using the exact same sim card...