Hello ZygimantasBliu,
what you are saying is not correct, because any TCP server can accept multiple connections on a single port, this is a native feature of TCP sockets: think of web servers, who only listen on port 80 (o 443) but they can serve webpages to hundreds of concurrent users.
Any commercial Modbus TCP gateway supports multiple concurrent connections from TCP masters, of course with some limits due to the compute resources available, but typically at least 8 or 16 connections are supported.
The gateway should simply send out the first request to the serial port and then wait for the device response (or timeout) before sending the next request from a different master, and so on in a round-robin fashion.
It should never break the TCP connection, unless the maximum number of concurrent connection is exhausted.
Regards