Fixing pfSense-Mikrotik site-to-site VPN

I need to have fixed a site-to-site VPN between pfSense and Mikrotik devices. The VPN connection works but Mikrotik's subnet is not reachable from pfSense's network side. Instead, everything works properly using OpenVPN client on a single computer. Moreover, I would like to have explanained what the issue is and the related solution.