Huawei Smartphones - why won't some work on AT&T?

Recently I've come across several different Huawei phones that were marketed overseas but brought to the U.S.:

P9 lite VNS-L31, originally Deutsche Telekom (T-Mobile Germany) - works on TMobile 3G, does not work on AT&T, no LTE (note testing done with Freedompop and FreeUp not the mothership)
Y5 MYA-L03, originally Tigo El Salvador but unlocked - works on T-Mobile and AT&T, gets LTE data on both
Ascend P7-L10 - works on T-Mobile and AT&T 3G, no LTE
P8 lite ALE-L21 - works on TMobile 3G, does not work on AT&T, no LTE (note- this one is dual SIM but only one SIM can access 3G/4G at a time)

Each of the phones seems to be unlocked. AT&T and T-Mobile SIMs are accepted. I've also inserted non-active SIMs from AIS Thailand and AU KDDI (Japan) and while there are no bars, I don't get a popup to enter an unlock code.

The Y5 I sort of understand. It was made for S.America but actually has U.S. LTE bands 2, 4, and 5. It doesn't have the higher bands but that's not all that different from even later model Samsung Galaxy flagships that are designed for Verizon.

I also sort of understand why the other phones work on T-Mobile 3G. Except for the Y5, the phones seem to have been made for Europe and don't have the LTE bands used in the U.S.

The part I don't get is why won't the P8 or P9 work on AT&T at all, even though they seem to be unlocked and have the radios to get AT&T 3G? And what distinguishes those two from the Ascend P7, which is actually an older model but does work on AT&T?

Could it be that AT&T has blocked certain newer models (but not older ones) due to pressure from the U.S.govt's "security concerns" re Huawei? But if that's the case, shouldn't the Y5 also be blocked?

Curious. I haven't seen anything that says the existing Huawei devices have been blocked from activation in the US.

I'm assuming that you've done a reset at some point, and have manually entered APN info...

APNs don't help when there are no bars whatsover. I simply get a blank and the no service indicator. With other phones, I would see bars and the 4G/LTE or H+ will pop up after configuring APNs.

And yes, I've reset several times.