I haven't run into that before, but I'll describe how I bootstrapped my setup in case that might help resolve your issue.
(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