bbs.blackview.com

It shouldn't be a compatibility problem. I can confirm that a Bv9900 Pro will work with 4G on AT&T in the US. Been using it for a few months now. It did however take a day or two to start using the 4G. I'm assuming it was just registering on AT&T's end. Depending on how old your SIM card is, consider getting a replacement one from an AT&T store. Mine initially didn't work and I had to remove/reinstall it to get it to behave. Mine had a good scratch in it so I decided to replace it, no issues since.

I found with Cricket, which uses ATT I had 4G most of the time in most areas. Not sure how ATT directly handles these international phones.

So I had the same problem as the original poster and was able to solve it by changing one of the SIM card slot's IMEI number to that of my old phone that was on the ATT network.
I changed the IMEI by following that link but with a few changes:

1. The SIM card is removed from the smartphone. *I did it with and without the SIM card in. With the SIM card seems to decide what slot you can write to. 
2. Dial in: *#*#8688#*#* and not *#*#3646633#*#*, into the dial pad (where you would enter in a phone number to call)
3. A screen should pop up with 'GPRS' across the top.
4. On the last line under "Write IMEI or MEID" enter in the IMEI you want to use. *It is a good idea to write down the previous IMEI in case you need to change it back.
5. Finally restart your device and verify that the IMEI changed by dialing in: *#06#

Important note, whatever slot you write the new IMEI to make sure to place the SIM card in the matching slot.

*Verify that the phone IMEI that you plan to use can get 4G LTE on the ATT network, otherwise the Blackview phone won't be able to either.