Hi Matus,
Thank you for your answer.
We can generate an offline update repository by using the Eset Mirror Tool, then we push an export of this repository to the server through an SSH/SCP connection (our only way to manage the server) and we expose it on localhost as an nginx site.
Too bad for the offline activation via console. It seems the only way to automate this step is scripting few browser's actions (chromium headless + chromedp) on the web console. I'll try this way and share my feedback.