Lost My Ability to Remote Connect

I had a jupyter notebook server running that I could access remotely until recently, and I do not understand what happened or how to troubleshoot it.

My server set up, with a Lets Encrypt certificate, is, I believe, exactly as described here. I blew away my .jupyter folder, ran jupyter notebook --generate-config, and copied over the essential information, but that has not worked.

A few other details. The notebook server works fine when I am sitting at the machine, and the security certificate is recognized. I do not believe it is network issue, as if I do nc -l 9999 on the server and then scan that port from another system, it appears to be open. My jupyter installation is via anaconda, and the versions are as follows:

jupyter core     : 4.6.1
jupyter-notebook : 6.0.3
qtconsole        : 4.6.0
ipython          : 7.11.1
ipykernel        : 5.1.3
jupyter client   : 5.3.4
jupyter lab      : 1.2.6
nbconvert        : 5.6.1
ipywidgets       : 7.5.1
nbformat         : 5.0.4
traitlets        : 4.3.3