Well last night I went between spectrum and roku, roku was clear as I watched newsmax, one time very briefly I thought I might have seen a little pixelation but when I switched it back to spectrum cable, it was horrible, lots of dropped audio and pixelation during the tucker carlson show.
Spectrum app no longer available on Roku! If you have it fine, but you cannot download it anymore until they finish their spat.
Think of the butterfly effect. A little data corruption coming into the house is them compounded when it’s decrypted in the cable box. That is why you would see worse crap through the box, rather than just coming through the modem. There is less processing.
Curious, what quality resolution are you watching? If your TV is not set to the proper resolution, it might be trying to keep up with the stream coming in. Streaming sticks (Roku, Amazon Fire, etc) process everything at the same resolution. 1900 pix will process slower than 4K. Pixelation happens when the color of the pixel changes. If the refresh value of the TV is not the same as the stream, it could cause some issues.
If your equipment is capable, try streaming some 4K content, or at least 1080p HD on your ROKU. If it streams those without issue, then the internet connection to your router may be OK after all and the pixelation a coax issue inside the home.
I’m a home theater hobbyist, and out of all the streaming devices, I like the 4K Firestick best. If you need to add Ethernet capability to the Firestick, this is the one to use:
https://www.amazon.com/gp/product/B07N2ZHFY9/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1