The best way to install and maintain .NET Core on Linux is to use Microsoft's package repository.
Instructions on how to do that can be found here (toggle the drop-down to select for different distributions).
So with Ubuntu 18.04, for example:
$ wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb
$ sudo dpkg -i packages-microsoft-prod.deb
$ sudo apt update
Once the repo is in place, find and install the desired packages:
sudo apt install dotnet-runtime-2.2
sudo apt install dotnet-sdk-2.2