Hello @pikaille,
You can try to configure your proxy, so it forwards the requests to the document server. Please find our example configs for nginx and apache under the links.
Another possibility is to make access to the document server public and enable JWT token to secure the connection. Please find more information about JWT in our API documentation. You can set your own secret token in /etc/onlyoffice/documentserver/default.json, after that you need to enter the same secret token to the appropriate field in the Advanced server settings in Nextcloud administrator section.