I am definitely an amateur in networking, yet I'm our school's technology coordinator. I recently updated our Internet speeds to 400 mbps from Spectrum. We have 4 different buildings on our campus. 2 of them are right next to each other, but the rest are maybe 300 feet apart.
We're having some major speed issues in the buildings besides the "main building" where the modem is. When I plug directly into the modem, I receive the 400 mbps speed. When I plug into the router in the main building; however, I get about 70 mbps. All of our buildings are wired, so we have everything wired through the WAN ports of that main-building router right now. Once you get to the external buildings, speeds drop down to 40 mbps or even 30 mbps. We have classrooms full of kids that are trying to work on Chromebooks, but the speeds prevent them from doing so.
I'm looking for ideas on how to improve the speeds coming out of our routers. I'm using a combination of Netgear R6400 and R6700 routers which I believe are more than capable of handling 400 mbps speeds. Is the issue coming from the distance between buildings? I thought that originally, but after getting slower speeds from my first router, I'm not so sure that's the issue. Maybe I should be using a switch in the main building instead of a router?
Just to be clear, here's a "diagram" of our setup right now (all connections are CAT-5):
Modem --> Main Router in school office --> other building routers/printer/office computer

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic