EDIT :my answer below refer "Maximum Server Connections" while you probably talked about  "Maximum Connections". AFAIK  "Maximum Connections" is for accepting client connections, how can it prevent maintaining connections to server ?
I have set Minimum Server Connections = 1 (before it was 3) and Maximum Server Connections = 256 but it does not seems to make a difference.

When used with NSD, I set Min = 255 and Max = 256 (I don't remember why you need 1 spare connection)

Setting No Activity Timeout to 5 (the minimum possible) and checking Apply timeout to all connections also make very little difference.

In System Status, the Idle time / Timeout column will goes to something like 67/5, then 70/5 and so on (increasing every seconds and showing timeout is exceeded) but it does not close the connection at all.

Apache server and IRIS have been restarted.

I think this will affect mostly outside network connections, what I described is about the internal connections being held between Apache and IRIS Server, through CSP Gateway.