|Please post to help the OP and not about rambling reasoning that does not offer a solution to the problem or really any specifics at all.|
That point aside, the proper setup would have been to use a vlan switch to segment the network and not ip subnetting.
Given the equipment at hand the correct setup would be as follows;
Router A is the guest network
Router B is the business network
No Router C.
A is connected to the internet. B is connected to A.
This results in both having internet but those on A can not get to B.
I am curious as to how you badmanrc are determining under the current configuration that B can access C and visa versa?
Technically 192.168.10.0/24 subnet can't talk to 192.168.15.0/24 subnet without a router providing the path. SOHO routers have no routing on the lan ports.
Answers are only as good as the information you provide.
How to properly post a question:
Sorry no tech support via PM's