Hi, when I connect my ethernet cable through my fiber modem I have no internet access, but when I connect the cable directly to my router it works fine. It has worked well before but sometimes, maybe once every month, it stops working for some reason. I get faster download speed when I am connected through the fiber modem (400Mb/s vs 800Mb/s). Please help!
Your modem may need to be reset to accept the different MAC (hardware) address of your PC vs your router. I don't recommend directly connecting to a modem for an extended period. Your PC has a public IP address when connected to the modem and is open to direct attack from the internet. Your router protects you.
The speed difference could be configuration on your router. Do a factory reset, with minimal configuration -- admin password, WIFI SSID and password. Then see what your speeds are. With 800Mbit, you won't benefit from QOS or other things.
Routers that can handle above 400Mbit are a small set. What router do you have?