Blackview BV9900 Pro No Reception

I have VERY POOR reception on my Blackview BV9900 pro in this entire area of New York State, and NO reception in my own house.  (I get 1 bar of 3G outdoors.)

I am with Consumer Cellular, which uses the AT&T infrastructure.

I have been on the phone with Consumer Cellular for almost 2 hours (so far); the access point names needed to be changed, but that did not help.  I have also working in the steeple of my church, which is FILLED with cell antennae of various carriers (including AT&T), but could not make calls, even though I had 4 bars' reception.

I downloaded an app to this phone via WiFi to test signal strength; it reads between -90 db to -140 db, and usually -110 to -120 db.  I don't know whether it's actually the phone or the (distant) cell tower.

I had no such issues with my iphone 7, which was on Verizon; but both Blackview and Verizon have told me that the 9900 pro is not compatible with that carrier.

I have tried calling Blackview in China, but I can't get through.  They have not yet answered my e-mail.

What else can I check, and what settings can I verify?

MANY THANKS!!

---Brad