Stretch to Buster upgrade from local nonfree DVD iso

I am trying to upgrade from stretch to buster using the nonfree installation DVD ISO "firmware-10.2.0-amd64-DVD-1.iso" that I downloaded using the torrent here:

https://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/10.2.0+nonfree/amd64/bt-dvd/

I copied the ISO to the local drive on my computer and added this line to fstab:

/data/firmware-10.2.0-amd64-DVD-1.iso /mnt/apt iso9660 user,loop 0 0

mounted it, then added the source like this:

sudo apt-cdrom -m -d=/mnt/apt add

which added a line to /etc/apt/source.list, but when I do "sudo apt update" I get the error

"Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs"

then

E: The repository 'cdrom://...snip...' does not have a Release file.

N: Updating from such a repository can't be done securely, and is therefore disabled by default.

What am I doing wrong?

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