I googled your problem and found a lot of information:
Make sure that the IOMMU controller it’s enabled
xHCI handoff - Enabled EHCI handoff - Enabled IOMMU controller - Enabled
Don’t forget to disable UEFI and change all boot options to “Legacy Only”.
also someone said that you should use “iommu=pt” instead of “iommu=soft"
Also Here says:
Go to your Bios “supr” or del” when you turn on your computer, and when in Bios look for “network stack” and set it to “enable.” (also tells you to install the realtek driver)
Do you have IOMMU enabled in the BIOS? Try booting with “amd_iommu=on” appended to the boot options.
Looks like you did it wrong, just restore the original grub file (/etc/default/grub) and add this at the end of it:
# Disable iommu
Then run this command to rebuild grub:
sudo grub-mkconfig -o /boot/grub/grub.cfg
And that should do it
"This board is not directly compatible with Linux, specifically the built-in Ethernet. In order for it to work you must enable IOMMU in BIOS, but disable IOMMU on the linux kernel command line with “iommu=soft”. Without this all will appear working, but you will have absolutely no connectivity. This bug / compatibility issue affects all revisions past 1.1 that have the new UEFI BIOS."