Comnect with Q3A ysing your dreampi. At the end of the connection sequence it should say "connected at 193.168.x.x"
That will be your dreamcasts ip. Id try dmzing that first, since it seems you can ping google.
Youll need to login to your router, goto WAN settings and look for anything regarding DMZ, should be a single field where you can enter 1 IP address. Thats where you enter your dreamcast ip. Once you do that reboot your pi and try connecting again.