HUAWEI MU609 Mini-PCIe card is available at aliexpress.com for about $40. Comparing to DW5550 card, MU609 is more expensive, but it”s a current hardware, actively supported by the manufacturer.
MU609 supports the traditional PPP interface, as well as CDC Ethernet interface for Linux.
It also has a built-in support for mobile voice calls, but its audio is only available on the physical PCM GPIO interface, which is wired to pins 45, 47, 49, and 51 on the Mini-PCIe plug. These pins are not standardized and marked as “reserved” in Mini-PCIe specification. The PC Engines APU board does not connect these pins to anything.
The card initializes 5 serial-USB devices (ttyUSB0 – ttyUSB4). ttyUSB0 can be used for modem control with AT commands. Detailed documentation for the rest of devices is available at Huawei website. The CDC Ethernet card is initialized as eth3 (because eth0-eth2 are onboard Ethernet adapters).
Setting up the card for automatic startup under Debian:
apt-get install -y picocom ppp cat >/etc/chatscripts/sunrise.MU609 <<'EOT' ABORT BUSY ABORT 'NO CARRIER' ABORT ERROR TIMEOUT 10 '' ATZ OK 'AT+CFUN=1' OK 'AT+CMEE=1' OK 'AT\^NDISDUP=1,1,"internet"' OK EOT cat >/etc/chatscripts/gsm_off.MU609 <<'EOT' ABORT ERROR TIMEOUT 5 '' AT+CFUN=0 OK EOT vi /etc/network/interfaces allow-hotplug eth3 iface eth3 inet dhcp pre-up /usr/sbin/chat -v -f /etc/chatscripts/sunrise.MU609 >/dev/ttyUSB0 </dev/ttyUSB0 post-down /usr/sbin/chat -v -f /etc/chatscripts/gsm_off.MU609 >/dev/ttyUSB0 </dev/ttyUSB0