I'm going to make vpn via a public IP. When I plug the cable directly in the mikrotik port, and set the IP, It pings and can be seen from inside the network, not from ouside whereas it can be seen from both inside and outside of the network if the cable is connected to a computer. I wonder whether I should make any especial settings in the firewall, router, or anything else?

ip public is on modem ?

No, it goes directly from satellite dish to poe

I'm guessing that you don't have anything in the firewall blocking this at the mo?  Some providers block inbound traffic to IP's distributed so it might be worth checking with them before digging around to much.