Unable to use functions from gsm_ppp driver

[ivonalo]

Hello everybody,
as described on this link: #48625 when using the muxing off ppp and at commands on my modem I am unable to send my custom AT commands because the gsm_ppp API keeps sending AT commands to the AT channel constantly without locking. That is why I decided to modify the gsm_ppp driver myself. But the problem is that I am not allowed to use functions from that driver in my application code and I don't understand why. I am able to compile the changes in gsm_ppp.c and gsm_ppp.h files and include the gsm_ppp.h file in my application code. But when I try using some of the functions that are declared in the gsm_ppp.h file I get an undefined reference to that function. Has anyone any idea why is that happening? I saw that there is a sample in the zephyr repo which uses that gsm_ppp driver and calls functions from the driver so I guess it must be possible? When trying to build that sample I also get various errors