You haven't clearly stated what kind of Sonic Internet service you have.
Do you have traditional DSL or Fusion ?
The reason I'm asking is that you mention modem rental which I thought only applies to Sonic Fusion but you bought Cisco 678 modems which are for traditional DSL (ADSL1). Such a modem can be used on Fusion (single line) but will not get the full speed.
Furthermore the fact that you bought a pair of Cisco 678's makes me worry that you intend to use them on bonded (dual-line) Fusion and that is 100% guaranteed not to work. Bonded (dual-line) Fusion service requires a dual-line modem.
Edit: Regardless whether your service is traditional DSL or Fusion, Sonic does not use PPP, PPPoA or PPPoE. RFC1483 is a specification that covers multiple ways of encapsulating LAN protocols (not just IP) in ATM cells and therefore choosing RFC1483 alone is not a guarantee to selecting a compatible mode. If there are separate options for "RFC1483 bridged" and "RFC1483 routed" then the former is the correct option.
RFC1483 was obsoleted by RFC2684 but the older name is still found frequently in DSL modems.--
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