I would start with getting a good idea of how things actually work, like the TCP stack, DHCP, Wifi, etc. Learning these basics will help a lot in diagnosing issues, because you'll be able to isolate it to specific parts of the network to look at. Having a good grasp of the basic principles of networking has really helped me a lot in figuring out issues.
A lot of the other stuff - like "how do I configure wifi in the office" - is highly vendor-dependent. The process for, say, setting up a guest network is going to be different depending on what hardware you have. But if you have a good grasp of the basics of networking you'll be able to figure out what those settings actually do.
Sadly, I learned a lot of this stuff through trial and error and long frustrating attempts at getting wifi and routing and VPNs to actually work, so I don't have any materials to recommend.
Edit: Another tip is to be able to build models of the systems you're making so you can test changes. It's incredibly helpful to have an environment you can break and rebuild quickly to test things. This goes for basically everything sysadmin related.