Managing usw on ucg behind 3 party isp router.

I'm a bit of a UniFi networking newbie, and I have a UCG Max with a Cisco 1100 router connected downstream on port 4.

The Cisco 1100 is managed by a third party, and I don't have access to it or the ability to make any UniFi-related changes on it.

Behind the Cisco 1100, I have a USW-24 switch. Devices on that side receive DNS/network settings from a server behind the Cisco router.

I would still like to be able to manage the USW-24 from my UCG Max/UniFi environment. I've tried creating an isolated VLAN from the UCG to the USW without DHCP, but at best I only get an unstable connection.

Does anyone know of a foolproof way to set this up while keeping the Cisco router in place and untouched?