I have the router TD-8840 from TP-LINK and it works well, but disconnects after some minutes it is idle.
How can I disable that feature? I want that it will be connected allways.
I don't have any experience with the specific device but most commercial routers refer to this option by something like "Nailed up connection".
If this device doesn't provide such an option you could create a cronjob which runs every minute and creates some -minimal- traffic in order to not let the connection die.