If your CP4/5 has recent firmware, it will be running a multicast DNS (mDNS) service. When you press the Find Mount button in APCC, APCC does a mDNS query on the network and lists any response(s) it receives from listening CP4/5s. Because APCC's query is sent to the multicast address of the local subnet, only CP4/5's on the same subnet will respond and thus be listed. But most modern managed switches and routers offer the ability to reflect mDNS queries and responses across subnets, so you can have your APCC-running computer on one VLAN (and thus subnet) with the CP4/5's on a different one and still utilize the Find Mount feature. Routers/switches generally need such a mDNS relay service turned on, though.

But if you want to manually punch in the IP of the CP4/5 you're looking to connect to, that works as well. Same VLAN (IP subnet) or not doesn't matter assuming the hosts involved have the requisite routing in place to reach each other, any network ACLs permit it, etc.