After upgrade tp 2.9 serial console was forced to serial0 even if before uprade serial0 was used by modem.
I tried to move serial console to serial1 and elave serial0 free but MT then reported that both serial0 and serial1 were occupied by serial console.
After I rebooted MT, everything came to normal. Serial console showed up only on serial1 and I was able to assign serial0 to modem again.
Did it all through Winbox.