I have the same issue. Local channels have NEVER been available even though I have both internet and tv packages. I'm also connected on my home network and had my Roku setup since Feb.
I've called and the operator couldn't supply me with an explanation. Completely unexceptable and frustrating....
If you do not or cannot, for whatever reason, find CBS or ABC programming on your Spectrum Roku app, there is another solution that works just as well. All four major U. S. TV networks, including PBS, have standalone apps that will allow you to stream their programming; however, the CBS All Access app is a paid service, while the others are absolutely free. Most other cable networks have streaming apps as well, so in theory you wouldn't need the Spectrum Roku one to get network TV programming.
You can also get every major U. S. television network's programming via Youtube TV and Hulu with Live TV, to name but two. Bear in mind, however, that to get live TV with Hulu you must have a subscription to the service, and PBS programming is presently unavailable on either service due to financial and other considerations. To receive PBS programming on your Roku you must use that network's streaming app, which is available in the Roku Channel Store.