_kaurus can you check if your HDD or SSD is partitioned with MBR or GPT? MBR does not support Above 4G Decoding, but the symptom should be system lockup.

https://www.howtogeek.com/245610/how-to-check-if-a-disk-uses-gpt-or-mbr-and-how-to-convert-between-the-two/

Other then that I would remove one of the 1070's to see if the R9 390 got recognized. Or check if the system will accept 4 or more 1070's if you have more.