OBD STN1110 - Breadboard vs PCB Issue
Is it possible you have a difference in the method of grounding and cabling, such that there may be more Analog ground noise or Common Mode noise on cables?

DFT would warrant some method of loopback testing for analog output and inout signals with a test set and I am it not sure what capacity your simulation box has.

It may be tedious, but examining or simulating the signal source through your cables and verify the destination at the chip using high speed diff. probes might prove useful to check the input response and noise levels.

Use a logic analyzer or terminal in parallel with control characters visible and examine the results. If signal integrity is ok, look for invalid control bytes on the serial port. Using a diode OR with 10k to ground for RX @ TX , I used to display both on a single monitor to examine traffic in parallel. RS232 threshold is 1.3V.

Does AT@1 or at@2 give any Results? (not case sensitive)

Testability is a key part of the design phase often overlooked with hopeful results.