You must set yr router, use Virtual port or port forwarding (alternative). ie you need type cam ip address(internal..192.168.0.xxx) and use set port:80 and 554 (tcp/ucp).depend on yr ip cam setting.
The office computer : you need type real ip address (eg 119.2.24.xxx) and u can access yr ip cam, no need apply Dyndns.
Your mobile phone :normally u can dowload coreplayer to acess ip cam(different model of mobile phone have differenet method...).
Hope can help u!!