proxy_pass to dyndns address

[basti]

Hello,

inside a location I have a proxy_pass to a hostname with a dynamic IP
for example

location ^~ /example/ {
	proxy_pass https://host1.dyndns.example.com;
}

getent hosts resolve the right IP.
But in via nginx return a 504.

When I reload nginx it work until IP is changed.
The DNS Server for this is on the same host.
TTL is only 300s.

I have found the resolver directive, I'm not sure if this is the right
one because of the small TTL.
Is there a way to get this working?

Best Regards