Hello,
could you try setting the Slave ID manually? The listening IP should also be 0.0.0.0.
Also, I can see that your IP Filter rule is either misconfigured or missing. In the Modbus TCP over Serial Gateway configuration, open your instance, scroll down to the IP filter, select the Interface as WAN, and enter the IP address 0.0.0.0/0. This will allow all traffic to reach this service, and if it helps it can be narrowed down to the exact client IP for security reasons.
Let me know if this helps!
Best regards,
DaumantasG