Hi,
If use dynamic IP addresses on both routers, then you'll need to use dynamic DNS service. Although with FlexVPN you can define "tunnel destination FQDN" - this just resolves the FQDN to the current IP address and hard codes the tunnel using that current IP address, which would obviously cause an issue once the IP address changes.
Perhaps use DMVPN instead, as this allows you to use an FQDN to resolve the NBMA address. You would have still have to rely on a Dynamic DNS service, to provide the IP to FQDN mapping. Try this example which provides good information on how to set this up.
HTH
