It’d be great if you could send your actual Netlify URL so we can poke around your settings. In the meantime, I’ll share @luke’s guide to setting up branch deploys:
I know you said that your branch deploys are already working, but if you’re trying to redirect, say, staging.alias.com to staging.primary.com and those are branch subdomains instead of actual subdomains, you’ll want to revisit the real URLs of those sites. I believe they are more like staging--alias.netlify.com and staging--primary.netlify.com. I’m not positive that redirects like that will even work, but that’s where I’d start. And once you share you’re URL, that’ll help us further troubleshoot if you run into issues!