Hi Norbert,
unfortunately I can't help with the modem reset problem, but only about the Arduino reset one
.
You can try a software solution by disabling the HUPCL (hang up on close) flag of the virtual COM port, so DTR is not lowered after a disconnect.
The other 100% sure way is a hardware solution - you just need to remove one of the 100nF capacitors - details here
http://www.arduino.cc/playground/Mai...rialConnection
Also if your machine has real COM/LPT port you can just control the relay with it and avoid the whole Arduino thing. Since you already have external 5V supply for the USB hub with one transistor, a couple of resistors and a protection diode you'll be able to switch the relay directly.
Btw I am researching what 3G modem to buy and was looking into the ZTE MF626 but am a bit scared after this stories. It will not be my primary inet conection, so if it works for 1-2 hours will be enough but still ...
Any advice?
Regards,
Rado