multi-server WordPress site secure client/file portal

I’ve got a website running WP hosted on a remote server. I’ve been working on creating a client portal/file server off of that site. The concept is to allow clients to securely access files via the web so a client can upload or download a file larger than would be allowed attached to an email. The requirements would include that a given client would have a unique un and pw and upon logging in would only see files associated with that client. The one part of this implementation with which I’m struggling a bit conceptually is how to keep the WP site on the remote server and have that server and its authorized users securely access files on a local server. Thoughts? Thanks!

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