Is there a way to find out what devices on a network are connected to a specific gateway?

My company has two routers. An MX100 Meraki and an older Cisco 1921 router. We are in the process of deprecating and then re-purposing the Cisco 1921 router. The only reason the Cisco 1921 router is still around is for older systems that are harder to gain access to such as the HVAC system and security FOB system that have a statically assigned gateway that points to the old 1921 router. But we also don't know what other devices could be using the 1921 router as it's gateway. Services like NAT and DHCP are disabled of course and it only serves as a stop gap for the connected devices.

So my question is, is there a way to find out what devices on a network are connected to this router? I'm relatively new to networking so any type of suggestion would be great.