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

WANPIPE Multi-Port PPP support

configname: CONFIG_WANPIPE_MULTPPP

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

Uses in-kernel SyncPPP protocol over the Sangoma HDLC Streaming
adapter. In this case each Sangoma adapter port can support an
independent PPP connection. For example, a single Quad-Port PCI
adapter can support up to four independent PPP links. The PPP
option is supported on S514-PCI/S508-ISA cards.

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