If you have a DRM-free stream on a mobile device, you can cast from it.

If you have a DRM-free stream on PC, you can use Sunshine (PC) and Moonlight (webOS App, manual install) to stream PC to webOS. You can use webOS Dev Manager to install on TV from PC. docs

Maybe not possible in your situation but I have a SFF gaming PC under the TV, so can just use browser on that

You might get lucky just using the browser.

But unless you’re using pihole or some sort of DNS based ad blocking you will likely get smashed with ads.

There’s Kodi in WebOS brew appstore. I’ve only ever used Kodi for offline media libraries but as far as I know there’s plenty of third party Kodi addons for this kind of piracy.

Your phone and cast it. https://play.google.com/store/apps/details?id=com.instantbits.cast.webvideo