I use win-acme on one of my windows servers to maintain Let’s Encrypt certificate and it works just fine, however I use it for something different, not UrBackup. But I think you should get it working without any issues. https://www.win-acme.com/
I found a tutorial on how to get IIS working as a reverse proxy to sit infront of urbackup. LetsEncrypt has a few GUI based applications like CertifyTheWeb that can directly configure IIS with certificates.
I have IIS listening on the regular https port (443) and redirects unbeknownst to the user to the proper UrBackup port. Certifytheweb uses Namecheap DNS api to verify the name, issues a cert, and then applies it automatically.