Hello,
This seems certainly like something that can be achieved, however it is not called "Flexconnect". I am assuming that you configured the SSID without "User Controller for Access Controll" because this is the typical setup for corporate SSIDs. You need to check in the configuration which VLAN is configured as egress network of the SSID. You can select the respective AP group of the remote AP in the Network Tree, then go to VSC bindings. Open the VSC binding for the corporate SSID and check if a VLAN is configured as egress network. If a VLAN is configured the AP will send the traffic of this SSID with a VLAN tag to the network. The switch- port has also to be tagged in this VLAN in order for this to work. If no egress network is specified the traffic will be sent without VLAN tag in the native VLAN of the switch-port
Your issues may that you have different VLANs or different configuration of the switch ports in the remote site but all APs are in the same AP group and egressing the traffic the same way. You should check how is configured the switch to which the remote AP is connected. If the clients should obtain an IP address from the same scope from which the AP obtained an IP, then probably the SSID traffic should be sent untagged to the network, that means you shouldnt specify egress network in the VSC binding.
If you need an egress network (tagged VLAN) in the main office but dont need an egress network (that means you need the send the traffice untagged) in the remote branch, you can configure a separate AP group for the APs in the remote branch, then configure a new VSC binding for it without egress network.
For any questions you can consult the MSM Configuration Guide