I got a gigabit connection from my ISP (with PPPoE connection). Tested with the cable directly in the PC, works great, I get 1000 Mbps.
I bought an ASUS AC55U router, which claims to have gigabit ports, but if I put the cable in the router and connect the pc to the router, I only get 100 Mbps speeds. The connection status claims the speed is 1.0 Gbps:
I have a CAT 5E 4 pair cable, and as I mentioned before it works fine if I plug it in directly in the PC. All the drivers are installed, router has latest firmware.
I have no idea where else to look. I don't have any other gigabit routers to try.
Edit: visual representation:
with router https://i.imgur.com/PNXlQbX.png
without router https://i.imgur.com/J9KGlfi.png
Best Answer
Maybe your ethernet cable doesn't support a 1Gbit/s connection (physical problem).
Maybe you have to activate Gbit-LAN in your router settings. You can get there by typing in ipconfig (on windows) and type in your gateway address into your browser.
Good Luck.