With suggestion from joeqwerty, I took the suggestion of removing the ethernet cables from my Primary Router to the WAN ports of of my DD-WRT routers, and instead connecting them to standard ports.
Although this did allow communication of devices on the sub-routers, they also lost web connectivity.
After working with the settings, web connectivity on the devices under the DD-WRT was restored with the following changes to the DD-WRT routers...
- Setup -> Basic Setup -> WAN Connection Type -> Connection Type -> Disabled
- Setup -> Basic Setup -> Router IP -> Local IP -> (Choose a Static IP for DD-WRT Router, I chose 192.168.1.50 for one and 192.168.1.25 for the other.)
- Setup -> Basic Setup -> Router IP -> Subnet Mask -> 255.255.255.0
- Setup -> Basic Setup -> Router IP -> Gateway -> (Primary Router IP, probably 192.168.1.1.)
- Setup -> Basic Setup -> Router IP -> Local DNS -> DNS Server (I used Google's 22.214.171.124)
- Setup -> Basic Setup -> Network Address Server Settings -> DHCP TYPE -> DHCP Forwarder
- Setup -> Basic Setup -> Network Address Server Settings -> DHCP Server -> (Primary Router IP, probably 192.168.1.1.)
Bonus. There's also a setting:
Setup -> Basic Setup -> WAN Port -> Assign WAN Port to Switch
- Activating it would likely give an extra port to work with. (And likely would have made me not need to switch the cables). That said, I had already switched cables, and had plenty of ports, so didn't feel like bothering, but doing so is probably the better course of action in the long run.