We're here to help! Have you tried these steps to connect more than one YouTube channel to existing AdSense account: https://yt.be/help/1iV5 ? Keep us posted.
Post following these steps you'll be redirected to your channel's monetization page. You can now move forward w/ setting your monetization preferences. Once your account is activated--you'll see a message in your YT account letting you know the same (may take up-to 48hrs). (2/2)