Ok. I could be wrong and often am so, disclaimer given haha, but were your SSL certs for your Linux box hosting the Jot Server, created using the external.IP?

Typically SSL certs are created with a domain vice IP although it’s not unheard of in some cases. If not created with the IP, that may be one issue.

Assumptions ahead...

Do you have a spare domain to assign to that box and then to test, you can point DNS to that IP and once propagated, run Let’s Encrypt to get certs with that domain. Also I guess it’s possible to create certs with the IP  itself but I don’t know if Let’s Encrypt supports that or if there is any special configuration involved. 

Ok. So based on your updated comment I think you might be closer than you think.

Also yeah I had the same issue with my first hosts, not with node.js getting installed but with opening ports on the firewall.

So if you know for sure that the local server has 5100 open and is listening [ sudo lsof -i:22 ] 

and

If your certs are valid for the site you are hosting, then change https://externalip:5100 to https://yourdomaintothehostedsite:5100

If the cert is valid and the port is open you should (maybe?) connect or get a new error, at least that’s been my experience 

I haven't gone back through and read this whole thread but I assume you have this part of the config.json down solid?

/* If you have https server (recommended) you need to fill 3 fields below */
root: '/folder/with/https/cert/files',
cert: 'myfilename.cert',
        key: 'myfilename.cert',

It took me many, many attempts to get that exactly right.

Also your app.js is running and as a daemon? I ended up using forever so my statement is:  

forever -w --watchDirectory=/etc/letsencrypt/live/mydomain.name start app.js

sudo forever list ## to verify its running

Let me know what you find