HAOS on Fangtooth should work without issues.
Are you trying to add the NIC while creating the VM or after?
Do you have a bridge configured on TrueNAS?
HAOS will work without a bridge, but you cannot configure Home Assistant back-ups to save on TrueNAS unless you are using one.
Hopefully this post will help. It explains how I configured the network settings for my HAOS VM.