I'm having a problem with Network Manager, under Kubuntu 17.10, with Plasma 5.10.5.
Basically, I can't connect to my Netgear CM500 cable modem, without having to reset the modem, by disconnecting and then reinserting the power cable.

If I haven't done this, NM fails to find them modem, and I can't connect to the internet.

My network settings don't seem to be the issue here. Also, I connect directly to the modem from my networking port, with no router involved in my setup here.

Any idea on how to resolve this issue? I had considered asking Comcast to send a reset signal to my modem. Would that possibly help?

Of note is that I am working on a fresh install of Kubuntu 17.10, a few weeks old. I had decided to reinstall fresh after upgrading from 17.04 to 17.10, as I had a few problems that were bothering me. Although the fresh install cleared up those issues, it did cause this problem, which I didn't have after I did the upgrade the first time around...

This blog may be of interest to you...
http://aaron-kelley.net/blog/2012/01...-goes-missing/

It describes an automatic way of resetting a DSL modem when it is disconnected from the provider. It does assume that your modem model can be reached from your PC via a web interface though. The OP wrote a script to reset the modem when it disconnected from the internet. In your case, perhaps similar could be done be fore trying to start a PPPoE connection. (Other than that, this is really an issue with your modem and Comcast, so I'd be seeking support from them.)

When you connect a different device to that CM500, you always have to reset the modem. The modem learns the MAC address of the first device it sees after a reset, and won't recognize any other device. It's done that way to support a seldom-used configuration of modem plus a switch (not a router), where multiple devices on the switch can make up a LAN, but public IP addresses are given only to "N" known devices (where N is almost always "1" in residential service).

To avoid the need to reset the modem, you need a router, even if you're actually using just one device at a time. That way, the cable modem "sees" only the router's MAC address, and the router accepts whatever you connect to its LAN ports.