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.

L1S (l1_cmplx.c specifically) calls these 3 functions which are normally
defined in the DL protocol stack entity:

dll_dcch_downlink()
dll_read_dcch()
dll_read_sacch()

If we are building L1 standalone (without the G23 protocol stack), we need
to provide stubs for these functions.