Excellent, I'll look forward to getting to test this soon. My testbed will be DNSmadeEasy + Vultr + ProxYes, I already have update scripts for DnsMadeEasy & DuckDNS.org I can add NoIP later.
You would need to register your phones to that FQDN instead of the IP. The FQDN will float between the servers as needed.
While DigitalOceans floating IP is great, your backup server must be in the same data center. If you have issues with connecting to that data center both of your servers will be down. If you use a DynamicDNS and a FullyQualifiedDomainName you can have a backup in another part of the country or even world and the backup should work.