Huawei Phone IMEI issue

My husband and I recently bought new Huawei P Smart Plus phones in Ukraine (model #: INE-LX1), and are very happy with the phone's performance so far.  They worked perfectly while we were traveling and, after researching, discovered they should work on AT&T without issues. We went into our local store last night and things seemed to be going well; the sales associate and manager both tried SIM cards that were already activated in our phones, and they picked up signals and we were able to use mobile date.  However, once we tried to activate new SIM cards, they were unable to use the phones' IMEI numbers and were never able to figure out why. The phones would have access to make calls, but no mobile data. Has anyone else run into the problem or heard of a workaround?  I'll be happy to provide any more information or clarify if I've not been clear!  We really want to get these phones working on AT&T!