Searching in iOS Mail App is server based.
The search will be done per IMAP.
Does your IMAP server provide full text search?
This does also mean, that searching in Mail app does only work as long as you have an active connection.
E.g. in plane mode searching in the Mail app is not possible.

But you can use the iOS wide search to find emails downloaded onto your device.
This will always use full text search, not just in emails and it does not need any connectivity.