Name Server cannot ping on PPTP VPN

Hello ,
I created an VPN and my client can connected to my network and also can ping all my local Network IP but can not ping the name of computers . That mean my server ip is 192.168.5.1 and the name is SERVER i can ping 192.168.5.1 but cannot ping SERVER .

Please help me on this subject .

My Device is RB750GL and OS is 5.26 .

Thank you.