Clients can ping router, Clients can ping server, server can ping clients, router can ping clients, clients can ping router, router cannot ping server, server cannot ping router
In order for anyone to be able to help you, you need to provide A LOT more info on the network setup and at least the used devices.
What type are clients, server and router, what SW version, what ROS version, ... how are things interconnected ?
Is there something in between with "firewall" capabilities which might block this ?
Maybe even a network diagram, can be drawn on paper and scanned in.
I would say that the problem is the server as client<->router communication works.
No matter what software and hardware they are. Hope that at least one device has anything common with Mikrotik.