AT command interface works! The cause of the breakage was the same Nucleus API issue with NU_Create_Timer() which we encountered at the very beginning of this project with Riviera timers: the code in uartfax.c from TCS211 was passing 0 as the initial dummy value for the timer duration, and our FreeNucleus version doesn't like it. The fix is the same: pass 1 as the initial dummy value instead.

/*
 * I like using u8/u16/u32, but they don't seem to be defined anywhere.
 * So I solve the whole portability problem by defining them myself.
 */

typedef unsigned char u8;
typedef unsigned short u16;
typedef unsigned int u32;

typedef signed char s8;
typedef signed short s16;
typedef signed int s32;