When I install Debian 9 from CD, it did install with /etc/apt/sources.list pointing to CD.
To get this work properly, you need to edit the file
vi /etc/apt/sources.list
Replace content with
deb http://security.debian.org/ stretch/updates main deb-src http://security.debian.org/ stretch/updates main deb http://ftp.debian.org/debian/ stretch-updates main deb-src http://ftp.debian.org/debian/ stretch-updates main deb http://ftp.us.debian.org/debian stable main contrib non-free
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