How Do I Set Reverse DNS in the new Cloud Manager Interface?

First, you'll need to add an 'A' record for your domain and point this to your server's IP address. This can be done from our DNS Manager or with the registrar that you are hosting your DNS records at. Once propagation has occurred, you can then set reverse DNS for your domain.

From the new interface, you'll select the Linode you'd like to set this up on from the "Linodes" tab. Once you've selected your server, you can click on the "Networking" tab.

You'll now see your server's IPv4 and IPv6 addresses. To the far right of these addresses, you'll see 3 dot buttons that you will click on. This will allow you to add the domain name that you are setting up reverse DNS for and save the changes.

https://www.linode.com/docs/networking/dns/configure-your-linode-for-reverse-dns/