You're not doing anything wrong. I have a 65" LG C9. And nearly everyone at my company has an LG TV. I've never seen or heard anyone mention ads. My parents have used the same TV for several years.

When used with inputs such as cable box, Apple TV, or native apps like Netflix, Amazon, etc., everything works as expected.

I'm genuinely confused as to what people are referring to. The last ads I saw were in the Peacock app to watch Parks and Recreation (and they were surprisingly non-obtrusive mainly Apple and EV ads). But I didn't have a subscription--the ads weren't coming from the TV.

I purchased a high-end Samsung TV in 2011. That was my last Samsung purchase ever. I also had a Sony X900F, and suffice it to say, I'll never purchase another Android-based TV (the UI was slow unresponsive garbage, unintuitive, and user-hostile). The actual TV itself was of decent quality but hamstrung by its operating system.

In general, unless something has changed, stick with LG.