AltStore requires you to have a the AltServer opened on your Mac and your phone on the same WiFi to refresh. SideStore is branched off AltStore but allows to renew and install apps right from your iPhone (you only need a computer for the first install of SideStore iOS app, or if you to re-activate if you go past the renew date). I had AltStore before and SideStore is much easier to deal with imo.