I'm having the same issues you were when connecting alexmos via mavlink.
Wondered what settings you went with? I'm getting no response with my parameters
Here's my parameters:
channel 1 = rx-serial (using mrpro yaw and aux 3 pins connected to rx,tx)
sys id/comp = 1, 154
port = 115200, none partiy
send hearbeat = yes
Rc settings tab:
Roll-pin mode = sbgc serial_api
roll pitch yaw etc all set to no imput
SERIAL2_BAUD = 115
SERIAL2_PROTOCOL = 1
BRD_SER2_RTSCTS = "0"
( serial SR3 does not necessarily correspond to serial 3 just the 3rd Mavlink port enabled in order used (usb SR1, telemetry 1 SR2, telemetry 2/alexmos SR3)
SR3_EXTRA1 = 20
SR3_POSITION = 5
SR3_RC_CHAN = 20
all other SR3_ parameters set to 0.