What BIOS version did you use, did you change any BIOS settings?
I updated the BIOS to OBT8P320.bin as it was the latest available on the ONDA website: http://www.onda.cn/MotherBoard_Supports.aspx?id=369
D1800 BTC/ D8P -D3 PXE BIOS [BIOS]
Description: This BIOS universal D1800 BTC 6 card 8 card mine board optimization graphics card
is enabled by default LEGACY mode without disk
Updated: 2019-07-03
Still, HIVE OS detects all 6 cards but refuses to mine on the first 2 slots, only the last 4 slots mine correctly.