Hello Victor,
you should configure the C2821 to bridge between its lan interface and the ATM interface.
But if you are using IPv4 over ATM with enc aal5snap (RFC 1483 routed) you can face a problem with your provider.
In fact, by bridging instead of routing the 2821 will try to send out
IPv4/Ethernet /SNAP/ AAL5 PDUs (RFC1483 bridged)
when the provider expects
ipv4/SNAP/AAL5 PDUs (RFC1483 routed)
and no connectivity will result for this encapsulation mismatch.
So without a change also on the provider side you shouldn't be able to convert to RFC1483 bridged your ADSL connection
Hope to help
Giuseppe