I clicked the Help and Feedback link in my Creator Dasboard; then selected Get Creator Support. When one does that there are several options presented; select "Monetization and Adsense"; click e-mail Support. Place your Channel link in the required space, don't say that it is about a specific video, and in the space provided to describe your issue, say "Please enable non-skippable ads for my channel".
You should see them activated a few days after this. BTW, there is a thread with these exact same instructions in this section of the forum; I'm surprised you didn't see it.
Next question: what payment method are you looking for?