Of course the above is completely incorrect,...
Completly incorrect ? Really ?
You suggest setting up some VPN from private router to some external VPN server which offers users service of forwarding some particular external ports to particular VPN tunnel .... please show the example. Just one.
You can setup any outgoing VPN to an external host that allows incoming traffic.
You can install that host yourself when you cannot find someone who already does that (I would be surprised if that were true...)
When you cannot find it, just install a CHR in some virtual hosting service and use it as your VPN server.
Do you think that it is easy way ?
No, but that was not what you claimed. You said: There is no way to easy access it without help of ISP.
That is simply not true, you can get the help of other parties than the ISP, e.g. Amazon EC2.
It is not easy to setup but then it is easy to access it. So "there is no way" that is completely incorrect.
Easier than PUBLIC IP ?
Yes, because PUBLIC IP is probably IMPOSSIBLE for him! At least for IPv4. He is at a provider that hands
RFC1918 addresses to their clients, and believe me, those providers don't do that for the fun of it.
Even if we agree on your propostion then it means that such VPN ISP just remaps external ports to internal interfaces despite what the interface is. So we are at the begining: ISP has to remap external IP:PORT to internal one.
I don't know what you are trying to claim here, but for sure: a VPN can be run over such NATted connections,
when you select the proper type of VPN, without any cooperation from the ISP.
I do not say that you are "completely incorrect" with your proposal but you suggest to take a sledgehammer to crack a nut. However you still need nut-cracker to operate sledgehammer to crack in.
The question was "How to Access Mikrotik Router Configured with Private IP Address in WAN via Internet".
The answer is: by setting up some VPN to traverse that NAT hop that only allows outbound traffic.
Your answer is: it is impossible.
I don't agree.