TETHER can be split as TiE + HER, so remember it as TIE her WITH A ROPE.
Tether and Fetter are common because both words refer to a kind of restrain (rope or chain) for an animal.
tether --> tie+together
tether- tie+ her
Internet tethering in iphone means.. it can be used as a hotspot to connect or TIE other instruments or equipments to it.