Mobile carrier in the caribbean having Problems getting Iphones to work on our network.

We've tried contacting customer support, Spoke to senior representatives for hours on different days, all with no avail. The last rep said its above her head and is a corporate matter; can anyone help? We've put our apn settings in multiple Iphones and still cannot get them to work. We've recently bought Iphone 8 plus from Apple (unlocked for any carrier) and still have no luck. I'm hoping that someone can help us out or at least point us in the right direction. Any help we can get will be much appreciated , thank you