As instructed in the (old) User Manual I contacted Thecus Sales. Eventually, after some following up, the Thecus Sales Territory Manager answered that from May onwards this wireless function is not supported anymore.
From searching around I conclude that it is not straight forward to get a driver for Linux. Is there anybody who can provide instructions for selecting a USB 802.11n adapter for the N2200, loading the driver, and configuring its wireless settings?
Above USB dongle base on ZD1211/ZD1211b, which means following link model should supported : www.linuxwireless.org/en/users/Drivers/zd1211rw/devices , but Thecus would recommend choose the WEB camera form tested. (hlan: I think they wanted to write "one of the tested USB 802.11 adapters that are listed above)
I abandoned my search for an USB 802.11n adapter. The Linux kernel version required is 2.6.31 from which there is a chance for the related driver to be included - my N2200 runs 2.6.24 (ROOTCMD uname -a).
For an afterthought: I think that I should have considered a HomePlug Powerline adapter anyway for storing the N2200 away in a closet.