I don't use DynDNS but I do have two HAProxy servers, one locally and the other on a VPS. The VPS has a cron job that renews the certs every three weeks, and my local server rsyncs them to the right place every so often.
Then, on my pihole I send requests for my services to the local IP but on the same domain. Because the certs are looking at the domain name and not the IP the cert is valid both on my LAN and from the Internet.