I'll tell you how I learned, I got thrown head-first into a network switch design. I knew almost nothing about networking.
The most useful resource while I was learning was RFC1812 "Requirements for IPv4 Routers" [1]
Its an ancient document written the same year I was born detailing how future routers should be built on this relatively new thing called the internet. The language is highly approachable and detailed, often explaining WHY things are done. It is an awesome read.
To be honest you don't need to finish it. I only read the first few chapters, but I googled EVERYTHING I did not understand. The first few paragraphs took several hours. Talk to LLM's if you need a concept explained. Take notes. In a few days you'll have a very solid grasp.
[1] https://datatracker.ietf.org/doc/html/rfc1812