That is incorrect!
The ethernet port can have a local IP address like 192.168.0.2/24 with your modem having address 192.168.0.1/24, for example.
The IP address given by the ISP is not on the ethernet port, it is on the PPPoE interface in the router.
Besides that, it is no problem at all to have more than one IP address on an interface.
You will either have to add a route to 192.168.77.0/24 via 192.168.2.2 in your modem, or add
a Masquerade rule to your MikroTik (in IP->Firewall->NAT) with outgoing interface ether1.