default or selected kernelversion does not have config value CONFIG_WANPIPE_PPP.
Result is shown for kernelversion 5.4.123

WANPIPE PPP support

configname: CONFIG_WANPIPE_PPP

Linux Kernel Configuration
└─>Networking support
└─>Wan interfaces
└─>WANPIPE PPP support
In linux kernel since version 2.6.12  
Connect a WANPIPE card to a leased line using Point-to-Point
Protocol (PPP).

The PPP option is supported on S514-PCI/S508-ISA cards.

Say Y and the PPP support will be included in the driver.