Make sure your macOS version meets the prerequisites
https://learn.microsoft.com/en-us/dotnet/core/macos-prerequisites?tabs=netcore2x
If it does, then after installing via the installer, in a new terminal run this command
ln -s /usr/local/share/dotnet/dotnet /usr/local/bin/
Then try dotnet --version
hopefully that should work
EDIT:
You might need to add x64 like so:
ln -s /usr/local/share/dotnet/x64/dotnet /usr/local/bin/
Courtesy of Stan in the comments