This may not be so easy, what protocol does the ISP use over fibre? Honestly the network card that you will probably need might already pull more than the modem.

So containers have been standardized for a while now (OCI), and even if you install “docker” it’s actually just installing containerd with docker-cli. For years kubernetes is not even supporting docker-shim anymore. So there should be no issue. What is even the problem you are running into?