Hi Patrick
Have a look at this article.
Essentially you create the CSR and Private Key on Cisco IoS
Complete the CSR using a Let’s Encrypt Client (Certbot etc)
Make sure you download the certificate + chain in one file (this will install the intermediates needed for Lets Encrypt)
I have a cisco router with crypto image i can set up but it may be a while before i can complete the testing
Andrei