You may need to edit /boot/config.txt and disable enable_uart=1 and may need to add dtoverlay=pi3-miniuart-bt or pi3-disable-bt (depending on your preference). This either moves the Bluetooth to the mini uart, or disables it entirely. If you don¡¯t do this (and you aren¡¯t using Pi-Star which does this automatically), bluetooth will bind to the uart and your modem won¡¯t work.
Alternatively, you may need to use /dev/ttyS0 for the Jumbospot, I couldn't get /dev/ttyAMA0 to work and I also needed to disable the serial console, if you're using the standard Raspbian distro.