portfwding with dyndns on local ip

Hello Dear Friends

one of my client is accessing internet from 172.16.11.9 via 172.16.10.1 via my live ip

now they wanted to access their office pc from out side of city with help of dyndns ac registered on their router

when they ping to xyz.dyndns.org they dont receive reply if they open xys.dyndns.org in browser they are reaching my router os admin login page.

Please help me how to resolve this.

You can setup VPN server on MikroTik and client can connect as VPN connection and see all office network.