1) With a 510v5 I find that as soon as I forward port 80, either directly by editing the NAT through the web interface or by having a PC do it through uPNP lose contact with the modem. It still routes, works as a DHCP accepts uPNP request from uTorrent etc. but I can't access its web interface or Telnet CLI interface. This doesn't happen every time.
2) I find that DHCP does not work on a "new" internal IP address, e.g. adding 172.17.0.1 unless I first delete 10.0.0.138 before setting up the address pool. I.e. Add IP - Add Pool - Delete 10.0.0.138 never lets the DHCP work, whereas Add IP - Delete 10.0.0.138 - Add Pool does.