Update:
Gave my ISP a call and after initial raised eyebrows at my problem (modem/router disconnects to anything when I turn on uPnP) they suspected a faulty modem (according to forums the whole modem in general is faulty, but anyway...) and they will send me a new (different brand/model) modem, so yay... (anyway, hope I will finally be able to play games like ETW in MP with that one)
(Did you notice that I use ('s and )'s a lot (brackets they're called, right?)?)