How to find out if a port is open on the local network through a remote gateway?

Question

Computer networks

Good afternoon.
There is an external ip-address of the provider specified as an external gateway on the Cisco router.
There is an internal server with ip-address: 192.168.1.15 and open port 8080.
It is necessary to check from a remote machine - you can access through an external gateway with port 49001 to the local server with ip-address 192.168.1.15 and port 8080.
How can this be done?
Thank you.
Sincerely, Dmitry.
PS Port forwarding is configured on the router.

Answer 1

From local Telnet, from external Port check

Answer 2

nmap

Answer 3

first check if the tsiska is knocking at least