I don’t think there’s any sort of built-in antenna on the LTE XBee3. I was under the impression that the external antenna is absolutely required. I’ve never gotten it to connect without one.
We’ve bought ours from Digi Key. I think we’ve tried a couple of different models and haven’t had any problems with any of them. We’ve even put the 2G antennas on and had those work. https://www.digikey.com/products/en/rf-if-and-rfid/rf-antennas/875?FV=93404c3%2C1d6c0003%2Cffe0036b&quantity=0&ColumnSort=0&page=1&k=lte+antenna&pageSize=25&pkeyword=lte+antenna