Once you have admin access to a router, you have complete access to the WAN and LAN of that network. This is one reason that enterprise networks are (should be) segregated into multiple networks separated by firewalls. In that situation, all you would have gained access to is probably the "DMZ" or outer zone network.
With such a foothold, you can open up inbound and outbound ports, install monitoring on the router itself and use the router as a platform for attacking other systems further into the network.
Any enterprise router should have monitoring already installed to help identify this kind of attack, reporting it upwards to admins and the SIEM. A common scenario is for the SIEM to aggregate logs from routers to monitor for anomalies.