For anyone that finds this thread through Google - dell indicated that network cards aren’t supported in the X16 slot and they should be used only for video do not attempt to use a x4 or x8 card as it’s not guarantee to work.   I had to return the system .
Brand new Inspiron 3910 (12th gen I5 with SSD ) I inserted a known good working Intel 710-T2L network card, which is a PCI x 8 card, into the X16 slot. The system doesn't boot, just hangs on the Dell Logo.  I can get into the BIOS, and it shows that an Ethernet card is inserted in the Slot 3 (the PCIx16), but no boot up after. Any ideas? dell tech support won't help as it's a "third party" card.    
Inspiron 3910 - not booting with Intel 710-T2L network card