Youtube is a good start, or just browsing posts here on Reddit or elsewhere.
It does depend on your requirements, network, equipment etc for instance I did and could use a consumer based router, for Routing and Wifi. But this means all devices on one network, (Eck!), but also means I couldnt present a “Guest” network on a switch port to say present my work laptop a hard wired network connection but seperate from all my servers etc. So VLAN’s was the way for me, as such I went DD-WRT for a start before going over to SMB/Enterprise kit for Networking and then Routing. That then means also the same for Wifi AP’s, all meaning that consumer grade kit is out the window, purchasing SMB/Enterprise kit and the requirement for the Cabinet to house the main core equipment.
Start with the basics, how networks work, IP, DNS, DHCP etc, then once you understand that you can then start delving further. Originally (around 15x years ago) I had just a Laptop and Router, then a NAS (piece of poop WD one) which then quickly was replaced by a Windows Home Server, then the tinkering started which DHCP, DNS. Then a proper server with a domain, VPN Server, remote access. Then the more advanced Virtualisation, DHCP Failover, Dual Domain controllers, DFS replication, Veeam backup. Then the networking with VLAN’s, ACL’s.
Security is again another topic and is a question of do you or don’t you need access or restrict access from A to B etc. Such example is with the Wifi, without a Guest or VLAN’s Wifi SSID all those guests that come and want a bit of my internet for free would also be able to “see” my network, servers and computers. With that segregated network and ACL’s in place no device on the Guest network can see my kit, or talk to each other but just see the internet.
You can try and clear it by updating the firmware, failing that you can try one more thing which is a repair of the iLO install, however once it’s got to this state its highly likely that the NAND is now fubar. It doesn’t stop iLO working as I have a 2nd hand Microserver with the same issue and tried the fixes, but they didn’t work, and as such, it’s showing the error constantly. The only way to fully fix in the unfixable state is a board swap.