@Sam196201
For any content you want that is not available as a developed Channel/app in the Channel Store, you can try watching online if available and then cast or mirror to your Roku device, or use your laptop with an HDMI cable directly to your TV. (can even use your cellphone or other mobile device with appropriate adapter).
If this is the Anime Planet you are referring to (https://www.anime-planet.com/anime/watch-online/), then either method mentioned above should work for you.
You can also look into installing Web Video Caster to your Roku device (and on mobile device) if you want an easy method to wirelessly cast online/web content.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102