It appears that you have two RouterOS devices, one designated as “C” for client and the other as “S” for server, which has a public IP address.
Your need to establish a VPN connection between the two devices, it could be WireGuard/IPIP/SSTP… Once the VPN connection is established, you will need to apply a “mark-routing” mangle rule to direct all incoming traffic from C’s bridge to S. Additionally, you will need to implement a “masquerade” rule for incoming traffic on S’s VPN interface in order to enable it to communicate with the internet interface. This should suffice for your needs.