Connecting router to existing network causes problems

At our small business we have an existing network. We have tenants from other companies who may bring in routers and connect them to that network so they can have a mini-network and see all their PC's. But with this setup, the user with the router frequently has issues with the internet going down. Does anyone have suggestions on how to add a router to an existing network and have it work?

 

Should be no problem. Have their router receive its IP address from your network via DHCP.

 

We do, but still their mini-networks go down even if our large one doesn't.

 

I'll need to have more setup information.

How are these "mini" networks configured onto your larger network? Is the mini router getting an address from your larger network via DHCP? If it's using a static address, what is it? What are the gateway and DNS settings in your "mini" network router? Can you provide an "ipconfig /all" from a machine on one of these "mini" networks? Can you provide an "ipconfig /all" from a machine on your large network?