Hope this helps someone, as the official documentation is misleading.
When accessing OVH Public Cloud Archive via the scp/sftp/rsync gateway, the password is not a concatenation of tenant_name, username and password, but needs to have dots in between.
I think the safest way to get these values is to go to the panel, go to Cloud/Servers/
From that file, use OS_TENANT_NAME and OS_USERNAME, and the password you got when adding the user, join them with dots, and this should work:
scp filename [email protected]<region>.cloud.ovh.net:container/