Steam sadly disables the workshop in Offline Mode. This wasn't always the case, but some years back a Steam update changed this, and despite reports from players and attempts from Paradox and CO to get this fixed, it's still the same.
If you have the option to set up a hotspot with a smartphone and connect to it, then that's the most practical solution. Make sure to disable all auto updates for your games, then connect and launch Steam in Online mode. Disable your connection and start the game. It will give you a warning that it can't connect, but just ignore that. The game and workshop will work just as they should.
If that isn't possible, then you'd have to install all mods and assets locally, which is not ideal. It breaks the connection to the workshop, so you won't receive updates anymore, which means you'd have to manually check for and install updates regularly. Depending on how often you end up playing offline, it might simply be more practical to have a ...
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