Maybe you can try
sudo modprobe -r option
sudo modprobe -r qmi_wwan
sudo modprobe qmi_wwan
sudo modprobe option
And then maybe you can see the wwan0.
You didn’t modify the option.c yet.
Dear Bean,
its working know!
I edited again qmi_wwan.c , recompiled and - funily - removed option. Rebooted and its now working without an issues. Thanks a lot for your assistance, As I heared, it might be easier in the future https://www.spinics.net/lists/linux-usb/msg264889.html
Thanks a lot for your assistance and excuse my ignorance 
Glad to hear that.
I have post the patch here in How to use the RG255C with ModemManager and qmicli - #4 by AirMax.
diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c
index 9fe7f704a2f7..e9208a8d2bfa 100644
--- a/drivers/net/usb/qmi_wwan.c
+++ b/drivers/net/usb/qmi_wwan.c
@@ -1429,6 +1429,7 @@ static const struct usb_device_id products[] = {
{QMI_QUIRK_SET_DTR(0x2c7c, 0x0195, 4)}, /* Quectel EG95 */
{QMI_FIXED_INTF(0x2c7c, 0x0296, 4)}, /* Quectel BG96 */
{QMI_QUIRK_SET_DTR(0x2c7c, 0x030e, 4)}, /* Quectel EM05GV2 */
+ {QMI_QUIRK_SET_DTR(0x2c7c, 0x0316, 3)}, /* Quectel RG255C */
{QMI_QUIRK_SET_DTR(0x2cb7, 0x0104, 4)}, /* Fibocom NL678 series */
{QMI_QUIRK_SET_DTR(0x2cb7, 0x0112, 0)}, /* Fibocom FG132 */
{QMI_FIXED_INTF(0x0489, 0xe0b4, 0)}, /* Foxconn T77W968 LTE */