I don't get any guitar sound in headphones or monitors, but microphone is working great with headphones and monitors. Setttings for the guitar are as below:
Please check the GUITAR GATE parameter in the SETUP menu.
The Guitar Gate parameter sets a Threshold for the guitar noise gate. This means that any signal below this threshold will not be heard. If you set it to 0 dB, you simply cut the guitar signal, since it is the max level you can get. So set this parameter to a lower negative value, start i.e. at -60 dB and the guitar signal will be back. Then adjust this threshold to avoid guitar background noise when you are not playing.