Since you need to define username and password, I assume this is a DSL link and not pure ATM.
Therefore you should change the configuration for PPPoA, for example:
interface atm0
no ip address
pvc 0/38 (VPI/VCI should be configured based on ISP requirements)
encapsulation aal5mux ppp dialer
dialer pool-member 1
interface dialer1
ip address
ip nat outside
encapsulation ppp
dialer pool 1
ppp chap hostname
ppp chap password
ppp pap sent-username password
Best regards
Marco