You can download documentations (directly, or through apps like zeal or dash), e-books for your preferred topics, make local copies of videos (with yt-dlp) and from open source-projects. Just studying source code and documentation, and making some small projects of your own should help you by leap and bounds.

I used to spend several months with very little Internet access (every other week on my cell phone). I made a small server with Jellyfin, tons of podcasts with airsonic, several youtube channels, a big Kiwix library (Wikipedia and Wiktionary - several languages), an ebook library (I haven't found the right way to do it, Calibre-server is a little complicated). https://github.com/awesome-selfhosted/awesome-selfhosted

And also a dozen of physical books, typically in a language I'm learning.