At the moment, we no longer provide the dotnet base images for arm/aarch64 device due to some issues of recent dotnet version with QEMU since our base images are cross-built on x86_64 servers. You can check the available dotnet images here base-images/balena-base-images/dotnet at master · balena-io-library/base-images · GitHub. We are working on the issue but cannot tell for sure when the arm/aarch64 base images will be available again.
However, you can still use our Dockerfile to build the image yourself. This Dockerfile can be built on any aarch64 machines.