Does your isp support multilink PPP as not all do. Do you have necessary software to set it up? LinkUPPP Turbo from FCR is defunct and OS 9.1 was suppose to include MLPPP as part of OT 3.0, but if you've looked OT is only at 2.7.4
So yes, I use MLPPP and works with the Apple Internal V.90 and USB Supra v.90.
LinkUPPP was a $60 program but never got marketed very well.
Gregory