You must have an internet connection on your SI 2018 Server machine while registering.
You only have to register SI 2018 from one SI 2018 Client machine on your network.
You should have received your Product Key for SI 2018 via email.
The first time you launch a SI 2018 Client on your network and log in (same domain/workgroup where your SI 2018 Server is installed), you will be prompted to enter your SI 2018 Product Key or to register for a free 30-day Trial.
Registration is required after the 30 day trial period has expired. To register, click the [Register Now] button on the Home Page of a SI 2018 Client:
If you don't see the [Register Now] button on your Home Page, you can always go to Start->Help->Register:
You will then be prompted to enter your Product Key: