It is a compatible issue between the thunderbolt boxes and the Dell 7820 and Dell 7920 Windows 10 computer with Thunderbolt add-in card. You can check with the Dell to see if there have newer bios or driver available for the thunderbolt port. Otherwise you can use the shorter passive thunderbolt cable to fix this issue. The shorter thunderbolt passive cables have different behavior compared with active cables. But it can work very well with macOS environment using Thunderbolt active cable.