thanks to Unable to install latest x64 sdk on Debian Buster #1632
(You cannot use MS's Debian9 install guide. You have to install it manually, the good thing is you don't have to add MS's repository to your installation :-))
wget -O /tmp/dotnet_runtime_2.2.3.tar.gz https://download.visualstudio.microsoft.com/download/pr/28271651-a8f6-41d6-9144-2d53f6c4aac4/bb29124818f370cd08c5c8cc8f8816bf/dotnet-runtime-2.2.3-linux-x64.tar.gz
wget -O /tmp/dotnet_sdk_2.2.3.tar.gz https://download.visualstudio.microsoft.com/download/pr/5e92f45b-384e-41b9-bf8d-c949684e20a1/67a98aa2a4e441245d6afe194bd79b9b/dotnet-sdk-2.2.300-linux-x64.tar.gz