So going back to the title, what to study? Maybe some specific book? Private classes/courses?
Networking. If you want to understand the reasoning behind things this is where you start. A good foundation in tcp/ip, the 7 layer network stack, as well as basic network protocols (dns, dhcp, http, etc.) will go a long way toward helping you troubleshoot when things go wrong.
Maybe throw in some operating systems study as well for when you start to use docker.