First of all from the remote PCs check wheither they can ping you host server, if they can reach the host server then they should not face any problems to reach the server. I think you are using the real ip address for the server. Do let me know the brief network topology you are using.
You cannot map drives or ping between your server and your colleagues PC’s because you are not on the same network. Using Remote Desktop is essentially just letting you see the servers desktop remotely you are not actually creating a logical connection between networks. What you need to be able to map a drive off the server onto your remote PC’s is a VPN (Virtual Private Network)
Check the router your server is on to see if it can take VPN connections. If it can then set up VPN users on it and make sure your server and the 2 remote PC’s are all on different IP ranges then from the remote PC’s set up VPN connections into the router.