“if you have no internet connection you can still install software from the CDs.” I’d imagine it also refers to USB or whatever method of accessing the installation ISO. Are you using wifi? Is there any connection issue? Is there a way to plug in an ethernet cable (if not already being used)?
The fact that the distro and the documentation both refer to the ISO as a “CD” is antiquated and slightly misleading by this point in time.
You could try the .deb file from the Debian archives, but then it could have an issue pulling the dependencies unless you mount the USB drive to pull from.
Just dawned on me, but have you run “sudo apt update” and “sudo apt upgrade” before trying to install any packages? I mostly use synaptic or muon for installing software, but regularly update via the command line with “sudo apt update && sudo apt upgrade”. Without updating the repositories it might not be trying to pull from them (huge guess, never seen this problem). Either way, you want your packages up to date before installing anything new.