I have bought this device: USB2.0 WiFi 150Mbps N ROUTER - M1 | 3/4 G SkyShare. Product type H-G5, build time 2014, SW version 2.0.04 for a few USD price.
Name:  M1Portable3GWiFiHotspotIEEE80211bgn150MbpsRJ45USBRouter-SKUspanitemprop161427-10-800x800.jpg
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It is powered from microUSB port using cable (short, maybe 15cm) that is part of the product package.

After connecting the device to your computer, you can access the administration via web browser:
Default IP 192.168.100.1, default user/password: admin

First you should change your admin password.

And in Network > Internet settings you define the desired operation mode:

1) 3G Mode - can convert USB modem 3G into home internet (i have not tried this yet)
2) Wire Mode - works as a Wi-Fi access-point. Connecting it to the main home router using classic network cable (UTP with RJ45 connectors) and other devices will be able to connect this small router via wifi. Default wifi password is 12345678. You should change it to something difficult and long to prevent abuse of your internet.
This small router has in default settings around 3 times lower signal than my big router. In same room i was able to download 1MB/s (megabyte) and upload 2MB/s.
When i was behind the wall, the speed was like 300kB/s. Behind two walls, it was maybe 10kB/s.
3) Wireless Client Mode - works as a Wi-Fi client which connects wirelessly to your main big wifi router or access point and then converts the signal to the RJ45 connector/network cable which can go to your computer. That way serve as a wifi dongle.

For the Wifi AP operation (Wire Mode), i suggest selecting WPA2 PSK mode for security and AES encryption. Other options are claimed to be weaker in terms of security.

The USB slot can be used likely for the USB flash drive or the 3G router modem. The router has a FTP server apparently.

Overall the strength of the wi-fi is really for one room use or for rather slow internet behind the wall. Or for two rooms distance without walls. Device is worth the money and i could also power it using my smartphone charger rated for 500mA output and 5V.