Cannot install dkms from ISO on a fresh Ubuntu installation with no internet connection : E: Unable to correct problems, you have held broken packages

I just freshly installed Kubuntu from a KUbuntu 20.04.3 LTS ISO.

The wi-fi is not working yet so I need to install dkms to add, build and install the rtl88x2bu driver.

But strangely it won't install although it is a fresh installation.

My KUbuntu 20.04.3 LTS ISO is mounted in /media/iso :

$ mount | grep iso9660
/iso/linux/ubuntu/amd64/kubuntu-20.04.3-desktop-amd64.iso on /media/iso type iso9660 (ro,relatime,nojoliet,check=s,map=n,blocksize=2048)

And my apt source is pointing to it :

$ grep /media/iso /etc/apt/sources.list
deb file:/media/iso focal main multiverse restricted universe

Now here is what happens :

$ sudo apt-get update
Get:1 file:/media/iso focal InRelease
Ign:1 file:/media/iso focal InRelease
Get:2 file:/media/iso focal Release [2 792 B]
Get:2 file:/media/iso focal Release [2 792 B]
Reading package lists... Done
$ sudo apt-get install -V dkms
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
 dkms : Depends: dpkg-dev but it is not going to be installed
        Depends: dctrl-tools but it is not installable
        Recommends: fakeroot but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
$ apt-cache policy dkms aptitude
dkms:
  Installed: (none)
  Candidate: 2.8.1-5ubuntu2
  Version table:
     2.8.1-5ubuntu2 500
        500 file:/media/iso focal/main amd64 Packages
        500 file:/media/iso focal/main i386 Packages
aptitude:
  Installed: (none)
  Candidate: (none)
  Version table:
$ apt-mark showhold
$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
$ sudo apt-get autoremove
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
$ sudo apt-get install --fix-broken
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
$ sudo apt-get install -V dkms
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
 dkms : Depends: dpkg-dev but it is not going to be installed
        Depends: dctrl-tools but it is not installable
        Recommends: fakeroot but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Can you please me ?

Just found out that dctrl-tools and libdpkg-perl (from which dkms depends directly or indirectly) are not available from the mounted ISO (right after installing Ubuntu) :

$ apt-cache policy dctrl-tools libdpkg-perl
dctrl-tools:
  Installed: (none)
  Candidate: (none)
  Version table:
libdpkg-perl:
  Installed: (none)
  Candidate: (none)
  Version table:
$

After downloading them from another PC and installing them manually, I was now able to install dkms and build the rtl88x2bu driver for a functional internet connection.

I believe this is a KUbuntu 20.04.3 ISO bug here.