If a device with no signal scans a worker it has never successfully scanned before while online, the record page shows: "No signal, and this device hasn't cached this worker before — connect once online, then this record works offline too."
Why this is by design
FieldCred deliberately doesn't let a gate device pre-fetch a site's whole crew roster ahead of time. Rosters are never exposed to anonymous gate devices — there's no list to enumerate, which is a real security boundary, not an oversight. The tradeoff is that offline coverage is opportunistic: a worker who's passed through this specific gate at least once while online is cached and works fine offline afterward; a worker who never has, isn't, and can't be.
How to avoid it
Before heading to a site with known poor connectivity, do one pass scanning the day's crew while the device still has signal — even a weak connection is enough to cache each record. After that, the same crew can be checked offline for as long as the cache stays fresh (see the 24-hour staleness note in How offline mode works at the gate).