Were you buying the cheapest supported modem? Your modem was probably deprecated when you bought it.
Also, at least for Comcast, the fact that they don't support your modem doesn't mean it won't work. It just means they won't be pushing updates and if you call in with a problem they'll tell you to first replace the modem.
They don't seem bothered by the frequency ("is normal..")
Everything else connected to that part of the house seems normal.
The cable junction point out in the yard is usually under water... they replaced all that too, but said that's normal for this neighborhood (we're adjacent to wetlands). How that bombs out the modem inside the house continues to be a mystery.
I'm probably here for one or two more replacements, and I'll move to another part of my city.
This has been true for at least the last 5 years running.