SP 2016 on Premise & REST

Hi everyone.

I'm trying for a week to make my SP 2016 on Prem queryable with REST request from an external supplier.

The purpose for this supplier is to access a specific site and being able to read and retrieve new files in 2 libraries.

My configuration : I have the SP server, behind a IIS reverse proxy server.

What I did untill today :

- Created URL rewrite rule on reverse proxy's default web site ({REQUEST_METHOD} entry, ^OPTIONS$ template

- Set SPSecurityTokenService 's option ActivateOkResponseToCORSOptions to true

- Created Add-in with appregnew.aspx on the site

- Retrieved client_id, client_secret from this

- Went to appinv.aspx to set the wanted XML options to "/web"

- Retrieved the App Identifier

Today I don't know how to obtain a token for my supplier to succeed with his REST requests. I precise that I don't use (and don't want) any Azure things for the whole process, as I have my own AD.

From my side, my postman tests returns with 401 error.

From my supplier's side, he has a 90002 error (linked to... microsoft account accescontrol... Azure)

How can I make things to work (moreover without Azure)? Read lot and lot of docs, forums, but 

Thanks to everyone for reading and helping.

Best regards.