I spent 2 hours trying to figure how to get the Discovery Plus app on my TCL Roku TV & this is what you need to do.’
Sign into Roku on your cellphone via the Roku website. You have to go through the website or Roku app for some reason cuz the Discovery Plus app isn’t on the TCL ROKU TV yet. Once you do that you need to click the blue download link below next to ROKU.

This will take you to MOVIES & TV apps that are available. Scroll slowly until you see the Discovery Plus app & click to install it. Once you do that it will say “INSTALLED” then you go to your TCL ROKU TV & the app will be at the bottom where all your installed apps are. Click the * on your remote & you can move the app to wherever you want it.
Hope this was helpful! Have fun streaming!