You say now that you're a student so I have to wonder if you're a student with three 3D printers or you're a student at school. The typical school has a variety of firewalls in place to prevent certain risky traffic; their firewall could be blocking the Pi's access to the Internet.
Yes, one 3B per 3D printer is the suggestion. A 3D printer is a manufacturing device moreso than just a inkjet printer or similar. Think of it as a fancy CNC machine in a factory.
Okay, so you've confirmed that the Pi has been issued an IPv6 address (which we knew from before).
So it sounds like the wifi side of your network has a feature called a DHCP service which didn't give you a default route (gateway) or it simply didn't want you to have one. Plugging into the
eth0 network device with an Ethernet cable then got issued an IP address which did.
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 10.20.30.1 0.0.0.0 UG 202 0 0 eth0
default 10.20.30.1 0.0.0.0 UG 303 0 0 wlan0
10.20.30.0 0.0.0.0 255.255.255.0 U 202 0 0 eth0
10.20.30.0 0.0.0.0 255.255.255.0 U 303 0 0 wlan0
I've both plugged in an Ethernet cable and have connected a Pi4B via wifi to my 5Ghz zone nearby. I'm confident that my router is setup correctly because I am the awesomesauce.
In my case, I see two default entries listed under
Destination: one for each network adapter. You should have at least one and whichever it is should work.
Note that the
route -6 version of this has a lot more information and is decidedly harder to figure out. But again, you're looking for the line(s) which include
UG for the