How to configure a AX25 interface ?

[Jean-Christian de Rivaz <jc eclis ch>]

Hello,

I have successfully setup a AX25 interface and NM see it this way:
nmcli d
DEVICE   TYPE      STATE        CONNECTION
ax0      unknown   connected    ax0
eth2     ethernet  unavailable  --
ttyACM0  gsm       unavailable  --
lo       loopback  unmanaged    --
sit0     sit       unmanaged    --
wlan2    wifi      unmanaged    --

The ax0 interface have only a IP address (configured while creating the 
ax0):
ifconfig ax0
ax0       Link encap:AMPR AX.25  HWaddr MENHIR-6
          inet addr:192.168.222.16  Bcast:192.168.222.255 
Mask:255.255.255.0
          UP BROADCAST RUNNING  MTU:256  Metric:1
          RX packets:832 errors:14 dropped:0 overruns:0 frame:0
          TX packets:688 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:10
          RX bytes:74961 (73.2 KiB)  TX bytes:89851 (87.7 KiB)

How can I configure NM to add a static default gateway and a static DNS 
server when, and only when, NM have to use the ax0 interface ? This is 
on a Debian Jessie system, so there is no /etc/sysconfig directory.

Best Regards,
Jean-Christian