Maybe you should use an adapter 15pin to 9pin.

I don't know this modem.  The following answers assume it's the
same as my Wavecom 1206b modem.

Potentially, the 15-pin connector handles more than serial
comms.  It has pins to handle audio (voice calls) and a reset
line as well, as follows:

PIN     EIA     CCIT    Designation
1       DCD     109     Data Carrier Detect
2       TX      103     Transmit Data
3                       RESERVED
4                       MICROPHONE (+)
5                       MICROPHONE (-)
6       RX      104     Receive Data (out)
7       DSR     107     Data Set Ready
8       DTR     108.2   Data Terminal Ready
9       GND             Signal ground
10                      SPEAKER (+)
11      CTS     106     Clear to send
12      RTS     105     Request to send
13      RI      125     Ring indicator
14                      RESET
15                      SPEAKER (-)

My modem came with a Wavecom cable to adapt this to standard
9-pin serial, ignoring the other lines.

There should be a seperate connector (4-pin, only 2 of which are
used) for DC 5 to 32 volt power.  No UART or other serial
device is going to be able to deliver enough current to power
this device.

My modem came with an antenna adaptor for a standard FME
screw-on connector, as would be supplied with an external
antenna for a car kit.

I'm surprised you didn't get a set of necessary cables, adaptors
and instructions.  Mine also came with a CD of instruction
manuals, a roof-mount antenna and cable, a DC cable for car
use, and an AC plug-pack (which I think an American would know
as a "wall wart").

John