How to update nvt without Internet

I am trying to build an OpenVAS environment in an environment where I cannot connect to the Internet.
I want to update the definition file once every six months.
What should I do to execute the following commands in an environment where I cannot connect to the Internet?
/usr/sbin/openvas-certdata-sync
/usr/sbin/openvas-nvt-sync-cron
/usr/sbin/openvas-scapdata-sync

Moving into the correct category and closing as a duplicate of various other existing topics:

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