As I said before, for the short distances, & throughput involved, your old or new cables will not make any difference. By connecting it the way you have doesn't eliminate the potential Router problem. It serves absolutely no purpose. You need to connect directly to the modem.
When I searched against "es2251 modem specs" one of the things that came up was Spectrum Advanced Voice Modem User Guide
Under Reseting the Modem
Use the reset button to either reboot (power cycle) the modem or reset the device to factory default settings. When the reset button is pressed and held for 4 to 10 seconds, the device will reboot (power cycle).
This should cause the modem to assign an IP address to the connected device (Computer or Roku in your case.) Further down in the Guide under:
LED Behavior, there's a table.
Beside Ethernet (Internet) Left Status Light is says: Green Amber then it says
- On Amber – An Ethernet Device is Connected at 2500 Mbps (2.5G)
- Off – An Ethernet Device is Connected at 100 Mbps
Beside Ethernet (Internet) Right Status Light is says: Amber then it says
- On Amber – Data is Being Passed Between the EU2251 and the Connected Device
- Off – No link is established
So after connecting directly to the modem, and then rebooting the modem, pay attention it's Right Status Light. It should turn on (Amber) when you have a good connection. Ignore the Left Status Light as it will be off as the connection speed will be 100 Mbps.
Don't rely on the Roku's Connection Speed description of Good at 67 Mbps to influence you. Your picture from the Spectrum App still sucks and it needs more. If you can't get this sorted, maybe call your ISP (Spectrum) and get them to work on it.
I hope this is helpful.