Look up Let’s Encrypt and their tool Certbot. They generate free https certs. Though I’m less sure about if you can use it on dyndns? You may need to buy a domain name and CNANE it to your dyndns name. I like nanecheap for domain names though if you’re new to the concept.
There’s a rabbit hole of consequences and alternatives to what I just wrote though. Does dyndns hide your home IP? I’m not sure tbh