Hey @Ladislav_Cunderlik,
Thanks for the additional information. If possible, can you please test connecting your core directly to your router and see if you hit the same error message?
With that, did you follow the steps provided in the port forwarding article I sent in my previous reply? Here is an additional thread where a solution was made with fellow users on an ASUS TR router:
If you are not able to set up a static IP from your router, contacting your ISP would be the next step in getting a static IP set up.
I’ll be monitoring this thread for your reply 