No router no restarts, i have S also with upnp and upnp works there no problems, S 1 month 24/7 upnp port works no problems.
It is provider settings blocked i can only activate upnp.
I cannot put manual port open, so i must have upnp working, but rtorrent opens port only when starting after that maybe router blocks after 24h port, but rtorrent should then restart upnp port automaticaly or check self if port get closed, im not expert, i just know with S ds station it worked without problems.