Hi Patrice,
welcome to the forums (saw your other post on the same topic).
First question is probably, whether you are using the Rockstor on CentOS (soon to be legacy) or using the new RockStor on OpenSuse setup.
On OpenSuse I saw some reports that the nic is supported out of the box (in some cases under Leap 15.1).
If you e.g. look here:
there are some instructions on how to build the kernel module if necessary
Also, the github repo (of the driver provides some instructions in the readme file:
If you look inside the Asus provided Linux (64-bit) package (XG-C100C｜｜ASUS USA), you will find the same/similar instructions:
That might help you to run/install it.
Good luck