I answered my own question tonight. The 598U is a Sprint EVDO modem. In referencing all the different documentation and posts led me to believe that a APN name was needed. There is no APN required on the Sprint EVDO network. By leaving the APN field blank on the "General" PPP Interface tab, will stop ROS from executing the "AT+CGDCONT=1" command which this modem does not support.
The settings used to support the 598U modem on my 750G test router where;
Port=USB1, Data and Info Channels=0, Phone=#777, Dial Command=ATDT, Dial On Demand=un-checked. All other fields at default.
Hope this helps someone else.
Mike D