I've been reading this thread and wanted to respond yesterday but simply didn't have time. Now I don't need to, because you have been doing a great job Bryan.
I find it strange that an ISP would run a NAT, or block a port, that would stop the MC Access Key from working. I guess things get done differently in America. Anyway, I can help check if that is the issue without waiting for the Static Public IP Address to be set up.
Nemick, if you are happy to test this, do these steps:
0. Turn your firewalls back on, for your Mac-mini and on the router. Leave them on after this test as well. They need to be on, and they won't stop MC working, at least not without advising you.
1. Open a browser on any computer and enter the URL
https://www.grc.com/shieldsup Yes, it is a safe site. I guarantee it. It is a well-respected site for testing all things internet and security.
2. You can read the displayed page if you like, but it may just confuse you. You will notice that it knows something about you already. When ready, click the "Proceed" button.
3. On the next screen you will see an open data entry box between two rows of wide buttons, just below the words "You may select any service from among those listed above . . .". Enter 52199 in there, without quotes.
4. On that same screen click the "Lookup Specific Port Information" button.
5. On the next screen click the "Probe THIS Port" button.
6. On the next screen the important information is the Status column. It should be showing as Open, and with a red background.
See the attached image for what my results of the above show. The red xxx.xxx.xxx.xxx will show your external IP Address.
If the Status above shows either "Blocked" or "Stealth", then your ISP is the problem, and the Static Public IP Address might fix that. But you said JRemote used to work, and then stopped a couple of months ago. I don't know why your ISP would make a change that stopped you being able to access your MC Server from the internet, but maybe they beefed up their security. Unless you changed ISP, or got that new router, around the same time?
Given that you have now set up an IP Address Reservation for your Mac-Mini, and are Port Forwarding to it, if Port 52199 shows as Open in the above test, then JRemote should be working again. By all means test using the MC Access Key test method. It doesn't give much information when it gives a positive result though. See the second image below.
I would also test JRemote again just to be sure. WiFi off on your phone, open JRemote and try to connect to your Mac-Mini MC Server using the Access Key method.
I hope the above isn't too confusing. You are doing well working through the complex issues of networks Nemick. Keep at it. You are nearly there!