Weird issue with cable modem, speed, and connection

I have an Arris cable modem from Spectrum. It has 4 ethernet out ports. For many months i've been running two ethernet cables out from it, 1 directly to my PC 3 feet away, and one to the router for the rest of the house.
reason: if i go to the router, then out from that to my PC, i get about 65 Mbps. If I ran direct from the modem to my PC, I always speedtest at my full 120 Mbps. I gave up trying to figure out why my router was only yielding roughly half speed, and since a direct ethernet from modem worked, i left it at that.
This morning upon turning on the PC, i had no internet on my PC. I checked, and the rest of the house is fine. Now, no matter what, I can't get internet to the PC on the direct line from the modem. If I run a line from the router to the PC, it works, but still at ~50% speed
I have tried:

• all network reset commands (netsh winsock reset, etc)
• rebooting PC
• rebooting modem (twice)
• windows updates, incl drivers
• running network troubleshooter
• running ONLY direct line from modem to PC (pulled line from modem going to router)
• switched ethernet wires
• using 2nd ethernet in port on PC

by the way, months back, in trying to solve the 50% speed problem, I went out and bought a new router with all the latest standards, and to my amazement, still got 50% speed. So yeah, I have no idea what's going on.

If the cablemodem has 4 ports on it, that indicates that it also is acting as a router, so in effect, you have 2 routers.

If both of these routers are acting as DHCP servers and assigning addresses to your PCs, you might want to check the settings to make sure taht they are not trying to hand out duplicate addresses.

On the PC that was not working when connected to the cablemodem, make sure that you are connected to the cablemodem, then try to perform the following commands from a command prompt:

ipconfig /release
ipconfig /renew
ipconfig /all

After that last command, make sure you are seeing an IP address assigned from the cablemodem as well as a DNS server address and a default gateway.

Does the router have gigabyte ports if it only has 100 meg ports you can't get full speed

Samuria said:

Does the router have gigabyte ports if it only has 100 meg ports you can't get full speed

And, in addition, you need higher quality Ethernet cables to get Gigabit (1000Mbps) Ethernet than you do to get just Fast (100Mbps) Ethernet. Both cables from Router to Modem and from Router to PC would need to be the higher quality, CAT 5e or CAT 6 cable.

In order to get full speed out of the router, the cable modem should be placed into bridged mode which effectively disables the router portion in the cable modem and passes all internet traffic directly to the router without any type of NAT firewall filtering. That would likely be the source of your speed drop through the router.

Thanks for all the attempted help. Nothing worked. I finally got through to Spectrum and they believe the modem is faulty. I'll be switching it out tomorrow.