The latest update fixed my unstable issue. Maybe you can try default factory your phone.

You're welcome :)

I also tried with a PC. If I open any folder on the PC and type in the address \\192.168.0.1 on the top of the folder, it would show me the files I put into the USB driver, then I would be able to...

Have you checked whether your modem is in the compatibility list: http://www.tp-link.in/support/3g-comp-list.html?model=TL-MR3040 ? If the modem is not in the compatibility list, it is possible that...

Haven't tried such scenario. You may borrow a device from your friend to test if possible. Either the WAN port or the LAN/SWITCH port would be fine.

It sounds like an issue with the TD-W8901's wired connection. What's the firmware version and hardware version of the 8901 and WA901? You may try upgrade both of them to their latest firmware first.