To install the .deb file, you must use the dpkg tool within your OS.
APT install is for accessing .deb files contained within repositories, using Debians Advanced Package Tool. Your Linux distro comes with apt and apt-get, both of which are different from APT. APT supplies the packages, apt and apt-get call upon APT to supply the package.
DPKG is the installer.
So, your command is apt-get install - means apt-get call on APT to retrieve package and install calls upon dpkg.
To install from a local .deb on your computer, you need only call on dpkg directly. dpkg -i (package-name) - in your case:
sudo dpkg -i rtl8821ce-dkms_5.5.2.1-0ubuntu3_all.deb
Perform the above command from the Same Directory that the .deb package is in, otherwise it won't find it.
So, if you moved the .deb package into Downloads, then Change Directory to Download Folder first:
cd ~/Downloads
For ease- you can just move the .deb package right into your home folder, then open a terminal which defaults to home with ctrl+alt+t, then apply the above dpkg command straight away.
If the above .deb package is the only .deb package in that directory, you can save yourself some typing by instructing dpkg to install all .deb packages in that directory and since it is the only one, it will be the only one installed:
sudo dpkg -i *.deb
