Hello,
I'm sorry you are having issues with the device.
I have never configured the IP securitas software before so I cannot help you with that, but I do know PPTP and I know why is doing what is doing and how to fix it or work around it.
The reason why the router is forcing you to change the LAN IP when you add a new user is because it is trying to prevent future issues are you are using the 192.168.1.X subnet on your router, and since this is the most common subnet out there and you won't be able to establish a connection to the router if you try to connect from a remote network using the same subnet then the router forces you to change it to prevent conflicts.
If possible, it is definitely recommended for you to change the LAN to something different, it doesn't have to be the one the router is suggesting, but it does need to be anything else than 192.168.1.X.
If you are absolutely sure that none of your remote users will be connecting from the same subnet and you want to keep it the way it is, then what you need to do is to change your IP address before hand, add all the PPTP clients, and then change it back to the default subnet, this will definitely work.
I hope this helps, please let us know,