"How can I set up the Open DNS to monitor each router separately?"
The answer is simple: you cannot. Being able to do this, you would have to subscribe to Cisco Umbrella with Active Directory support and installation of local programs. But this is merely thought for businesses and corporations.
"I am asking that because I have 1 only public IP"
Yes, this is the reason why you cannot.
"is there any way where I can set up a private IP on Open DNS dash board to monitor and filter different conect on each router?"
Why do you think these IP addresses are called private? Because they cannot be seen by and are irrelevant for external services like OpenDNS. Fortunately external services cannot look into your private network (LAN) and its infrastructure. Would you be happy if they could? Or rather not?
What you can do is this: identify a similar service like OpenDNS, and configure OpenDNS on one router and the other DNS service on the other router. This should do what you want "to monitor and filter different conect on each router".