Sounds like certain Channels that were included with Dish you are no longer eligible to get free.
You may need to subscribe to a Premium service depending on what you want.
To add, I don't think that question about your TV provider really does anything other than install any relevant channels when you first set up your Roku. There's really no need to change your provider. If your provider has a Roku channel, just install it from the Channel Store.
Many cable-type channels have only made themselves available for streaming to those people who are paying for them via a paid subscription to a service (cable, satellite, etc.) that carries their channels. You typically have to authenticate this subscription to your "tv provider" when you first set up such a channel on your Roku or other device.
When you no longer subscribe to your old "tv provider" you are no longer paying for these channels, so you are no longer eligible to stream them unless you get a paid subscription to some other service that carries them. There are a number of such services that can be accessed via Roku: Hulu, YouTube TV, Sling, and others. It's a matter of researching which channels you want then figuring out which service(s) to which you'll need to subscribe.
One further caveat: any channel you subscribe to as an add-on to another service (eg. subscribe to HBO via Roku or via Hulu) typically will only be viewable when viewing on the service or device through which you subscribed. If you subscribe directly with the channel on its own website (for those that provide this option), you'll be able to view it on any device for which it is available.
The cost savings by "cutting the cord" generally come from subscribing to a reduced number of channels rather than overblown cable/satellite packages that contain hundreds of channels you never watch.
I have recently switched from Dish to Spectrum and I have the same question as you. I am a Spectrum streaming customer with predefined channels from Spectrum. The tv I am trying to reconnect my Roku stick on is a Sony. I can see through Sony that Roku is connected and I can see some apps as well. However, the Roku remote doesn't work and I cant see the Spectrum channels designated to me. I have 2 other tv's that use the Roku stick and I did have to select my service provider during the install. The difference this time is that this Song tv was connected to Dish and I had a Roku stick on it.Now I'm a Spectrum customer and unable to see The Spectrum designated streaming channels.
The question is, is a complete reinstall of Roku required and how do I do that? I've tried resetting the Roku, but that did nothing.
Yes, the only way to redo initial programming of the roku is by factory resetting it.
home> settings> system> advanced system settings> factory reset> follow prompts
We just figured this out.
For example, if you are wanting to stream something on Animal Planet. Sign out of Animal Planet and sign back in. You'll get a web address and a passcode to enter.