Meraki will automatically register a dynamic hostname with the public IP address it uses to reach the cloud backend. The dynamic hostname can be seen in the left hand column under "Security & SD-WAN" -> "Appliance Status"
If you have purchased a DNS name and would like to use that instead, you can do so.
In terms of reverse DNS, there's no specific requirement for it to be setup for the Meraki firewall to work.
For example, in one of my locations:
- A forward lookup of my Meraki dynamic hostname resolves to my public IP.
- A reverse lookup of my public IP resolves to a hostname registered by my ISP under their domain (not used by me in any way).