Thanx guys for the replies

@GrandPa, yes I checked already the port settings in FrostWire

@Lord of the rings, Yes I know you also need UDP ports forwarded, but I thought it wasn't necessary to get connected, only for best performance it's needed (I read it somewhere at the FrostWire website "For best performance UDP port is also needed, which made me conclude that it's not necessary:-)

But I will see how to also forward UDP ports in ISA Server