Restrict Session outside the LAN connection

Dear Freelancer, I am having one web-app and anybody who is having login credentials can login from anywhere over the internet but now i want to give login access to my LAN network only. It means nobody access my web-portal outside the LAN network. I am having Linux server and have cPanel WHM in it and the LAN's internet connection is with dynamic ip. Only for expert freelancers and for those only who can come on remote software for the required settings. Thanks