Once set up and installed, Knossos will be fully operational offline. After that, you can manually download FSO builds and link mods to them, as well as install actual mods by just downloading their files externally and placing them in proper directories in Knossos mod library, as well as getting mod.json files for said mods from somewhere, but that's not a big concern.
But before we start fiddling with all that, let me just ask you a question - have you tried using your phone as an internet access device? Back in the day I was in a similar situation and I was using the 'tethering' function in my phone all the time to access the Internet from my PC via a USB cable. There also should be a "hot-spot" function in basically every smartphone, setting up a wifi network with access to the Internet.
There's also an issue of what does your mobile data plan look like. With how chonky modern MediaVPs are, you might get into some serious internet access costs... I've updated MediaVPs just yesterday, and it was a 10GB download. Also Knossos tends to refresh its mod list file (>200MB at this point) quite frequently.