I am trying to replace my old Debian 7 on my laptop with a netinstall of Debian 9. I downloaded a mini Debian 9 (a few hundred MB) on a CD and tried that but it did not work, not connecting to my Vodafone "mobile broadband" gadget or unable to although the single user prompt came up.
I then downloaded DVD1 of Debian 9.2.1 (2.8GB) and copied it on a DVD but it does not boot, giving the message above. Obviously, DVD1 of Debian 9.2.1 is not up to the job although checking the mdsum had no complaint.

What can/should I do?

Thank you for your help.

how did you copy it to dvd?
have you checked debian wiki if maybe that requires some special process?

Thank you for the answer.
I downloaded the file as an iso and copied it from the "Download" folder as an iso to a DVD disk. However, I downloaded it through the browser which is not recommended because browsers are not good at recovering from a break in transmission in such cases although the file was downloaded without any problem and the MDSUM checked OK.
There is nothing special in the Debian Install Wiki which is from where I downloaded the file except that it is recommended to use preferably a wired network connection which is not possible in my case because the (only national wired) telco is unable to provide Internet access at my premises (consequently nobody else can), I downloaded through wireless Vodafone "mobile broadband".
There is a hint in the Wiki that the software on the DVD "may" not detect WiFi network which I find rather odd since whether it can or not should be a certainty. I could be wrong but I assume that "mobile broadband" is much older than Debian 9.
I should add that Debian 7 had no problem to "see" the wireless "mobile broadband" modem.

Copied? To create a bootable DVD disk you need to select the ISO file and burn as image in whatever tool you use to burn DVDs like K3B or Brasero etc.

ldlinux.c32 is a syslinux module. Its what makes the ISO a hybrid image i.e. boot from USB drive too.

commands issued.
download link.
full detail please.
while it is possible to use cp, the usual recommendation is to use dd.
but i don't know if and how it applies to dvds (as opposed to usb sticks).

That could have been my mistake. I'll try again.

My partner disposed of the CD/DVD so I have no way of checking what I had done but that seems to be what I have done: copy the file instead of burning it as an image, a silly mistake. It worked now.