Placing your modem into bridging mode will mean it is the responsibility of the Mikrotik to authenticate as well as get an IP. If the protocol of your ADSL provider is PPoA, you are out of luck. If your provider uses PPoE, then it will work by setting up a PPoE client on the Mikrotik and not a DHCP client.
You could also try placing the ADSL modem into 'half bridge' mode, where the modem does the PPoA authentication and you then get the public IP given to you by your ISP bridged across to the modem's ether interface.
We use a simpler method, an ADSL modem that performs the ADSL interfacing, but also translates the PPoA into PPoE. Therefore it is very easy to setup as the Mikrotik does the authentication as PPoE, and the Mikrotik's PPoE interface then gets the Public IPs.
nest - you are great, thank you for the explanation. It sounds right, I will look for the PPPoE authentication and then it should work fine.
THANKS !
Jakub