FTP/SFTP servers are not natively supported by the iOS, but can be connected to the Files app (and thus KeePassium) using third-party apps.
SFTP File Provider (a paid app) worked well in our tests. However, your mileage may vary.
There should be other apps that integrate FTP/SFTP servers with iOS. If you find something that works for you (or does not), please let us know.
Last Updated: 2019-06-20