I tried Files, but after some sort of timeout the File disappeared and I don't know what that was about, so I went back to Share.
(1) Setup Resilio sync on my NAS
(2) Shared the folder holding the kdb file via Resilio Sync
(3) Entered the code into the Resilio Sync mobile app
(4) Performed an initial sync of the folder to my phone
(5) Downloaded Strongbox and pointed it to the kdb file under the "Sync" location in Files on iOS