This issue could be due to the number of connections maintained between Enterprise
Gateway Server and the Internal Server, by default it is 5. Try increasing the Maximum Connections in Security > Ports > Edit Internal Server Configuration
Note: For best performance, set the Max Connections setting on the listener to be slightly less than the Maximum Threads of the Server Threadpool setting on the Settings > Resources page. If you have more than one listener defined on the Internal Server, the sum of their Max Connection settings should be slightly less than the Maximum Threads of the Server Threadpool setting. Do not set Max Connections to be equal to the Internal Server’s threadpool. Instead, reserve enough threads to handle the execution of scheduled services, triggers, and users that connect directly to the Internal Server.
Additionally you can also try to set the following watt settings watt.server.rg.internalregistration.timeout property to 180 and watt.server.rg.gateway.pinginterval to 60 in the internal gateway and DMZ gateway respectively