To the best of my knowledge it’s not a setting - merely PlatformIO detecting that you don’t have an internet connection, and skipping checking for updates.
I haven’t tried recently, but as long as all the dependencies are present (toolchain, libraries, etc), I didn’t have any issues with using PlatformIO offline / with no internet.
Is your scenario easily reproducible? i.e. does it fail to work with a simple blink program, on a platform that you’ve used before (so the toolchain files should all be present)? Or is it a project with specific library dependency requirements… i.e. the issues mentioned earlier seemed to partly be in conjunction with git managed depenencies.