Hello!
I am a Senior Network Engineer with more than 6 years of experience with project design and implementation in large Enterprise Networks. I have CCNA, CCNP and CCIEw certificates from CISCO. In this case you need to setup the DDWRT device as a switch and router, that means that PC-s wll still get public IP addresses and one of the ports can be used for the vlan 192.168.59.x, which will get to the internet with a source IP of 8.5.5.5. It is straight forward configuration, so please send me a message if you want to get it done.
Best Regards,
Ibra Ramku