Hello everyone,
I'm a relative newbie in networking but I still managed to set up my Mikrotik router when I bought it some years ago. So far I received internet from the ISP via UTP cable. This cable is connected to ether1 port which acts as DHCP client and receives a public IP. After changing the ISP, they gave me a router that acts as media convertor and then connects with my router via the same ether1 port. The Mikrotik still gets IP through DHCP, but this time this IP is private. The result is that I don't have internet on the LAN. There's no address overlapping, and I tried turning of the two "deny" rules in the Mikrotik's firewall without result. No settings were changed. The Mikrotik can ping sites by DNS. What could be the reason that I don't have internet on my LAN?

Could you export your config and post it here? I presume it may be NAT masquerading, but nevertheless

The proof is in the config for sure. Strange though all things being equal the config should work as the only change is dhcp client. Perhaps there is an overlap in that the private LAN of the upstream device is the same subnet as the LAN subnet behind the MT ??

The result is that I don't have internet on the LAN

And are you sure your new internet is working properly?
If you plug a computer/laptop directly into the new service does it get internet?