Hey @kayalan
The RTL8125 driver is present in CORE but disabled by default due to a data integrity issue when using iSCSI. If you don’t use iSCSI, then you can enable it by using the following method from the shell or CLI:
root@truenas[~]# cli
> system
system> tunable
system tunable> create var=if_re_load value=YES type=LOADER enabled=true
system tunable> create var=if_re_name value="/boot/modules/if_re.ko" type=LOADER enabled=true
system tunable> exit
Reboot after entering these lines, and your Realtek NIC should be functional.