Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can
encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint
links over an Internet Protocol network.

Here we are going to discuss the communication between public to private IP. (Cloud server and Mikrotik/NAS with private IP)

Example for CLI Commands – Ubuntu OS

modprobe ip_gre
lsmod | grep gre
ip tunnel add SC7 mode gre local (ubuntu server IP) remote (your router public IP) ttl 255
ip addr add 10.10.10.2/30 dev SC7
ip link set dev SC7 up

Mikrotik CLI

/interface gre
add !keepalive local-address=(core router public IP) name=SC7 remote-address=(cloud server IP)
/ip address
add address=10.10.10.1/30 comment=Synnefo interface=SC7 network=10.10.10.0