While R4.5 still does not support CHAP connections (which are used instead of PAP by some of the big major ISPs such as Earthlink), some ISPs may offer dynamic switching between CHAP and PAP. Many R4.5 users are finding that they can now make the connection by changing „Standard PPP“ to „Unknown Type 1“ in the Dial-Up Networking preferences panel.
Additionally, at least one user has discovered that you can edit /boot/beos/etc/servers.ppp as follows:

Unix_login T
 erid:s du assword? dw
 IV>s dppp
S

Then select Unix_login in the Dial-up Networking preferences to get an Earthlink connection working.

In addition, Ken writes:
Set the connection type in the DUN preferences panel to Manual, and enter your DNS server addresses.
Upon connecting, you’ll get username / password prompt. Earthlink accounts require a username format of („ELN/“ + Username).
Note that the backspace key will not work on the login screen, so if you mistype you’ll have to start over.