The D1 appeared as expected in my browser and showed the temperature and humidity.
I wanted this to show up in Home Assistant, which I had installed on my web server with Docker as I did not have an idle Raspberry Pi at the momemt.
Apparently to connect the D1 to Home Assistant I need to use MQTT, so I installed Mosquitto on the server which is running on CentOS 7. I thought that it would be a good idea to do this securely, so I set up port 8883 and used LetsEncrypt.
This is where things got difficult. I installed MQTT Explorer on my Ubuntu desktop and was able to connect securely to the MQTT broker with it. However, I could not with either Home Assistant or the D1.
A bit of research on the D1 finds that I will have to compile my own firmware in order to use MQTT and TLS. More to learn.
I have not yet found a succinct set of instructions to implement MQTT and TLS on Home Assistant either. There are a few "here's my configuration.yaml, try it" posts, but no one explains why they made the entries and how they work. Yet more to learn.