I don't have an official cherry device, but I'd imagine you need to flash official ROM first for it to recognise updates. (Unofficial ROM won't look for an official update.Thanks, lineage-15.1-20180830-UNOFFICIAL-cherry.zip is working.
how to update from lineage-15.1-20180830-UNOFFICIAL-cherry.zip to lineage-15.1-20180903-nightly-cherry-signed.zip by Software updater in settings.
No new updates are shown.
Flash full ROM from here: https://download.lineageos.org/cherry as you did with unofficial ROM, and then you may see OTA updates. PW.
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