How to enable ALPN support in nginx:
echo "deb http://nginx.org/packages/debian `lsb_release -cs` nginx" \ | sudo tee /etc/apt/sources.list.d/nginx.list
When you choose to install nginx from the official nginx repositories instead of the debian ones, you will get the newest version with ALPN support (Application-Layer Protocol Negotiation).
Just do an apt-get update & apt-get upgrade and you will get version 1.16.0-1~buster which includes it.
Here you can verify if it worked: