I assume that the servers have Private IP Addresses?

DrayTek routers consider all Public IP addresses to be insecure, whether they are physically connected to the WAN or LAN; and applies NAT and firewall between Public and Private IP addresses.

This means that:
– The firewall is NOT invoked when internet users connect to any device on your LAN which has a Public IP Address. You should therefore ensure that any PC with a Public IP Address is running a personal firewall.
– All communications between Public IP Addresses and Private IP Addresses are passed through the router’s firewall – even when both devices are physically connected to the routers LAN ports.

When your Public IP PC tries to ping or access a Private IP PC or server, the router’s NAT and Firewall are probably blocking the communication.  If you want your Public IP Address computers to access your Private IP Address computers you may have to set Firewall Rule(s) or Open/Forward Ports.