So, it seems you hit the jackpot - good job.
It also seems like the op has 2 network interfaces =- one on the new mainboard and one added from a transplanted card from the old motherboard?
Anyway, as long as he’s not using iscsi (which seems unlikely?) - could enable the Realtek driver following @Honeybadger’s instructions in this thread Install RTL8125 NIC driver on CORE - TrueNAS General - TrueNAS Community Forums