One of the things I struggled with initailly when using immutables was installing programs like VPNS that need to interact with the immutable parts of the distro but don't have a flatpak option. I figured I'd just make a post to help anyone with this specific issue regarding mullvad or if it helps people install other software they need.
Adding the repo
Jump into a location to download the repo file
cd Downloads/
Download the repo
wget https://repository.mullvad.net/rpm/stable/mullvad.repo
copy the repo file to the yum.repos.d folder
sudo cp mullvad.repo /etc/yum.repos.d
Install mullvad vpn
rpm-ostree install mullvad-vpn
Reboot to reimage
systemctl reboot
Join the client to the service
sudo systemctl enable --now mullvad-daemon
Install libappindicator that at the time wasn't included in Kinoite
sudo rpm-ostree install libappindicator-gtk3
Reboot to reimage
systemctl reboot
I don't know anything about immutable distros, but any good VPN provide Wireguard or OpenVPN config that you can just import into your network settings/manager. Mullvad does.