I found a very good article on how to do add SSL via Lets Encrypt to WordPress running in Amazon Lightsail. You can also use this if you’re using EC2. (Should the article ever disappear, it is archived here.)
I thought my site had been hacked. Suddenly, all content was gone and my FTP credentials were being requested.
To perform the requested action, WordPress needs to access your web server. Please enter your FTP credentials to proceed.