Once the certificate is in place, assign it to the DSM Web GUI. Under the “Configure” section in the Certificate tab, you can map the certificate to DSM services such as the Web interface, Mail Server, or File Station. Select the appropriate service and apply the new certificate. After applying the changes, your DSM Web GUI will be accessible over HTTPS. To test the configuration, open a new browser window and type in the address with “https://” followed by the NAS IP address or hostname and the HTTPS port number if you changed it from the default. Your browser should indicate a secure connection, and if a valid CA-signed certificate is used, there should be no security warnings.
https://vital-mag-net.blog/dsm-7-1-setup-configuring-the-web-gui-to-https/