What resources do you recommended on learning about routers and router security?

Depending on your background, you might want to take a look at something like the Cisco CCNA material that covers basic setup, operation and networking. It's a great primer and there are plenty of free resources available. Here are some popular learning resources.

Router Sims like - https://www.boson.com/netsim-cisco-network-simulator

Book - https://www.amazon.com/Routing-Switching-Complete-Study-Guid...

some links that might be helpful.

Router Security Checklist (contains a lot of links/references): https://routersecurity.org/checklist.php

RouterSploit - Exploitation Framework for Embedded Devices https://github.com/threat9/routersploit

you may or may not want to look at recent research papers of SDN, Data/Control/-Plane security.