Remote Access to RentalDesk NX

You can't run RentalDesk NX over the internet unless you have direct access to the machine that hosts the RD Server and can ping its IP address.
This often means setting up a VPN (Virtual Private Network) connection between the remote machine and your office network, a task that can be quite time consuming and tricky to achieve.
However, even then, the performance of RentalDesk NX would be far too slow for everyday use due to
i) the amount of data that needs to flow to the client machine for the software to process,
ii) the TCP/IP latency over the internet introduces quite a significant overhead because of the sheer volume of commands being send back and forth between the RentalDesk and its server.

A better solution is to use remote access technologies (Remote Desktop, GoToMyPC, etc) to get access to a workstation that normally runs RentalDesk NX on your network from your remote machine. Essentially, there are two implements of this:

• The cheapest and simplest way is to use remote desktop technologies like Microsoft's RDP (Remote Desktop Protocal) or GoToMyPC in order to effectively hijack your office machine and remote control it via your out of office computer. If your office machine has XP Professional or Vista installed then RDP is built in, but in order to protect that machine from hackers you would normally still need to set up a VPN, which is not always convenient. 
  
  Dedicated remote access software like GoToMyPC gets around all the complexities VPNs and allows you install software on the host machine that uses alternatively technologies to make the machine available (securely) even via a web browser in any internet cafe. There are many other competitors to GoToMyPC that offer similar solutions so you might want to do a search for alternatives.
• If its not viable to take over an office computer remotely because someone else may be working on it at the time, consider deploying Windows Terminal Services machine on your office network  that acts as an application server, allowing multiple user sessions having remote access to it and run applications as if they were running on the desktop of their out of office computer.  Terminal Services also relies Microsoft's RDP, so a VPN connection might still be required. Talk to your IT advisor for more details.