If you want to dive into the "why" and "how" of networking, Cisco has a great training program that will go deep into the protocols. I am a current CCNA and CCNP and both have helped me immensely in my career - and surprisingly with MikroTik. Understanding how networking works made it a lot easier for me to transition to MikroTik when I first started working with it years ago.
If diving into the CCNA or MTCNA is a bit too much, I would recommend working through the protocols that interest you using this site:
The labs are setup for Cisco, but I've had new engineers on my team complete them in EVE-NG using CHR (or Juniper, Nokia, etc) to quickly learn protocols.