Hello,
Please Help Me with this,
Introduction and Story:
Firstly… I am putting this out there: I am an electrical engineer not a network guru or not even close to an enthusiast. Did took the CCNA exams flawlessly about 15-18 years ago, but since I do not work in the field I kind of forgotten most of what I learned there. 
At a friend recommendation I bought this Mikrotik router RB4011iGS (that positively impressed me - compared to what I had before – some commercial router from Asus ). It took me about 2 weeks to configure it as best as I could (IPv6 took me the longest) and then forgot it for a long time, in the closet where most of the IT stuff is (like: some cables, the ISP modem, a Phillips Hue modem and the welding machine).
In the last 2-4 weeks I bought a CCTV System. I checked my DDNS (apparently named Cloud – took me a while to find it ) service and it was already up and running. I was glad. I configured a new NAT rule, forwarding a specific port to the CCTV NVR IP on other port (to make it interesting and probably safer), and then I thought to myself (after testing ofc) that everything is done and I can forget about it for another 3-5 years.
But then… this happened: Tried to connect to my CCTV System from my phone while driving back, to see where I can park my car on the street, where I live. And then… surprise… got no connection. Got home, checked CCTV system… was running fine… connected to it via local LAN all was fine… tried to connect to it via Phone Tower (again)… did not worked. Interrogated the DDNS to see if I get my external IP address from my phone and seemed to be working.
And now… closer to the problem at hand:
Got this error: “Your router is behind a NAT…” or something like that (in the cloud service windows). The only NAT that I have is on the Mikrotik router itself (the ISP does not block anything), so I disable all NATs and even the Firewall rules, tried again… did not worked. Soo, after a lot of tinkering of trials and errors, one idea popped into my head and I rebooted the router.
And… it worked! Eurika! I no longer get the error message. I re-enabled everything (in baby steps ofc) and still no errors. I can now connect to my CCTV again from anywhere in the world and see what happens around my house and even if I have a free spot to park my car in front of my house.
Question:
What can I do to not get this problem anymore? … I want to forget (again) about the router and I want to be sure that next time I try to log in, I can see my CCTV system.
Thank you in advance,
Best regards,
Me (Manuel)