I'm aware that this subject was already widely discussed here on this forum,
That is where the problem starts and it goes downhill from there. You made the assumption that what you are thinking is understood because you had read some previous posts. To put it bluntly, you have done a poor job of explaining what you need to do and what the problem is that you have. I have been trying to fill the blanks and figure out what it is but, as I have stated, I don't feel I have an understanding of what you are doing and why.
Now it seems that you have two routers as I had guessed before. And you want to disconnect the outside connection to the remote server. And I have little confidence that what I just said is true.