Do you have a block of public IP addresses assigned to your site from your ISP? If yes, did you configure the WAN interface of the Mikrotik with a public IP? If you don't have a public IP address block, you can still access the Mikrotik by first configuring its WAN with a static IP address that is a part of the ONT's LAN network but outside it's DHCP range. Next you configure a port forward in the ONT to point to the static IP address on the WAN of the Mikrotik. Use www.whatismyipaddress.com
to determine your ONT's IP address. You can then use a service like DDNS to resolve your IP address.