How to learn more about routers?

What exactly is it about a router that you want to learn? Network protocols? Ethernet/WiFi drivers? Actually designing the electronics and sitting with a gigahertz range oscilloscope to test the signal integrity of your optical transmitters?

If it's just networking you want to learn, you can get quite far with just virtual machines. Build a distro like openwrt for x86, run a bunch of qemu instances, put them all in various bridge configurations, or maybe even play around with ovswitch. Sky's the limit.

The simplest thing that might work is any old computer with an ethernet port and one of these:

https://en.wikipedia.org/wiki/List_of_router_and_firewall_di...

Evaluate OpenWRT source code.