What is your redirect rule? Can you show a screenshot?
You don’t have a DNS record for the www.sandbox.app.thecampstack.com subdomain, and you should be prompted to create one after creating your Redirect Rule.
As for the DNS record for the www.sandbox.app.thecampstack.com subdomain, what should the settings for the record be? What should it point to? I wasn’t prompted to create one.
Just replace yourdomain.example with sandbox.app.thecampstack.com and keep using a 301.
You weren’t prompted to create one because you didn’t specify the subdomain on your rule. Your rule should have redirected everything (*) to itself. I’m wondering why it didn’t work.