How do I reinstall NetworkManager in CentOS 8 while offline?

You will need to download the packages locally, but since your networking is not working you will need to download them while in Rescue Mode. We will put them into /media/sda/packages which will correspond to /packages once you are booted normally.

From Rescue Mode:

mkdir -p /media/sda
mount -o barrier=0 /dev/sda /media/sda
cd /media/sda
mkdir -p /media/sda/packages
cd packages
curl http://mirror.centos.org/centos/8/BaseOS/x86_64/os/Packages/NetworkManager-1.32.10-4.el8.x86_64.rpm --output NetworkManager-1.32.10-4.el8.x86_64.rpm

There may be more dependencies needed, but we will find out about that later. For now, reboot normally and go back into your LISH Console.

yum clean all
yum remove NetworkManager --noautoremove
yum localinstall /packages/NetworkManager-1.32.10-4.el8.x86_64.rpm
systemctl enable NetworkManager.service
systemctl start NetworkManager.service

You will want to reboot once more. If networking is still not working and you are seeing missing items in your /var/log/messages file, you can find the missing packages and dependencies on this site (you will want to go through the steps above for downloading and installing the missing packages):

https://centos.pkgs.org/8/centos-baseos-x86_64/NetworkManager-1.32.10-4.el8.x86_64.rpm.html

I hope this helps!