When setting up new routers?
If you create a new router:
- you can select an external network if you want your new router connected to the Internet (you can do the same later by a "set gateway" command). If you don't want - don't select external or don't set gateway.
- you can connect your router to one of your networks - by adding a new interface: then you have to select a subnet to which you want to be connected and then DO NOT select an IP address for the interface but allow system to set it by default.
In current version of the system setting an IP address by yourself will end up in your new router unavailable and dysfunctional.
Unfortunately this means that you cannot have two routers on one subnet.