My goal is to establish direct TCP connection with SARA-U201 (all UART data is interpreted as RX/TX from the TCP socket). But I would like to have API to retrieve GNSS data from the GPS module as well. I try to achieve it with the CMUX protocol using 2 DLCs. Channel 1 would be the direct TCP connection, Channel 2 for AT commands to reference the GPS. I used the gsm0710.c implementation as a reference for the MUX implementation and overall it seems to work - I can communicate with HDLC wrapped data with 2 separate DLCs. But when I try to TCP connect in MUX mode, SARA-U2 doesn't respond with anything (AT+USOCO=0,"ip",port). The same sequence of commands works in direct mode.
Is there limitation of what can be used in MUX mode or it is just some defect in my implementation?