> a while ago our local ISP jews cut out all cable internet in the city because "muh 5G is the future" and I'm forced to use a retarded mobile internet dildo.
That sucks. I wonder if it's possible to use a router that has a 5G modem so you can just plug in an Ethernet cable to your PC (or use a Wi-Fi AP) and let the router itself handle the 5G?
> Of course I have an internet connection - on my Windows machine that is
Hmm, you could use VirtualBox (https://www.virtualbox.org)
and install Linux on a VM. IIRC, you can "mount" VHD virtual disks using Windows. You can also drag-and-drop (or was it copy/paste?) files into/out of the VM, If you install the VirtualBox guest addons:
On Arch/Artix the package is called virtualbox-guest-utils (you also probably need to install base-devel, dkms and linux-headers)
On Debian/Devuan the package is called virtualbox-guest-x11(you probably need to also install build-essential and dkms)
(NOTE: on Debian virtualbox-guest-x11 package is only available in unstable/sid release, on Devuan you can get it from ascii-backports and unstable repositories)
If your distro/version doesn't offer the virtualbox-guest-x11 (or similar) package, you can use the Virtual Box software to insert a virtual CD that (automatically) contains a installation script/files for VirtualBox guest addons (to do this, you just need to click on a menu item in one of menus on VirtualBox window. Note that you still need to have build-essential and dkms installed before you attempt to execute the script inside the VM.
> So unless someone can teach me how to download libx11-dev as files and offline-install it to a separate Linux machine,
It depends on your distro but at least
all distros that are based on Debian/Ubuntu and Arch have a mechanism for installing packages offline.
On Debian/Ubuntu, you can use the DVD(s) as a package repository: https://help.ubuntu.com/community/AptCdrom https://linux.die.net/man/8/apt-cdrom
If you have access to a Debian PC that has Internet Connection: https://manpages.debian.org/unstable/apt-offline/apt-offline.8.en.html
On Arch/Artix, you can just download the pkg.tar.zst file from one of the mirrors and copy it to a memory stick, and then you can install it using pacman -U ./foo.pkg.tar.zst (just like when using AUR+makepkg) On Debian there is similar apt command, but I have forgotten it. The reason why I know it's possible is that Debian doesn't include proprietary drivers by default and I had to manually download the firmware.deb file and copy it to a flash drive and manually install it using apt (nowadays I just use Devuan which includes the non-free firmware by default) You can find/download .deb files using the Debian package search site: https://packages.debian.org/bullseye/libx11-dev#pdownload
(you also need to make sure you have its dependencies also)