[WWAN][RM500K][MT6880] Is it possible to support a RNDIS 5G dongle with 2 interfaces?

[Macpaul Lin <macpaul lin mediatek com>]

Dear NetworkManager developers,

A new type of 5G Modem dongle has been added to support list to Linux by 
this patch.
https://lore.kernel.org/all/20220623085644 13105-1-macpaul lin mediatek com/

It use usb-serial as a control path running tradition AT commands.
The data path is RNDIS (ethernet) interface.

Quote the description from the kernel patch above:

"RM500K provides 2 mandatory interfaces to Linux host after enumeration.
 - /dev/ttyUSB5: this is a serial interface for control path. User needs
   to write AT commands to this device node to query status, set APN,
   set PIN code, and enable/disable the data connection to 5G network.
 - ethX: this is the data path provided as a RNDIS devices. After the
   data connection has been established, Linux host can access 5G data
   network via this interface."

I've tried to configure /dev/ttyUSB5 as a 'gsm' type device with 'nmcli' 
for setting 'apn', 'pin code', and other AT commands. However when I 
tried to up this interface. 'nmcli' will show 'cannot find the device'.

Is it possible to add a new kind of setting type for this dongle and 
leverage the origin 'gsm' and 'ethernet' configuration logic?
If there is any hint or indication to the configuration logic that will 
be very helpful and thankful. For example, should we add a new type for 
this kinds of device? Or should we add the configuration logic under 
'gsm' type?

Thanks
Macpaul Lin