Add Tether
Open this page on your iPhone and tap the app you use. It opens and asks to add Tether's source. Then find Tether in Browse and install it.
Nothing happens? Add the source by hand: open Sources, tap + and paste this.
https://github.com/viktor-svirsky/tether/releases/latest/download/source.json
Don't have SideStore or AltStore yet? Set it up first, see how to install Tether.