amaccuish
Hi,
as far as I know on the iOS devices the user is able to select where to search (everywhere, just the subject, just the sender, just the name of attachment). grommunio-sync then performs the full-text search, there's nothing to enable.
It's also possible that the iOS filters the results and displays the items matching search criteria in the FROM field only. I don't own an iOS device and am not very familiar with them though, so I'm not able to say if there's some configuration option on the device itself.