If you have issue(s) with the built-in driver, you can try this open
source driver from github.
Plug in the EW-7811Un to your system's USB port (2.0 preferred),
making sure your system has temporary Internet connection
Open a Terminal program and make sure your system is up-to-date
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
Note: You may need to restart your system if a kernel update has been installed.
Install the necessary packages to compile a device driver:
sudo apt-get install linux-headers-generic build-essential dkms git
Reboot, then clone the github repository:
git clone https://github.com/pvaret/rtl8192cu-fixes.git
Note: Right after "rt", it is a letter "l" (like Larry), not a number "1" (like 1980.)
Set up as DKMS module:
sudo dkms add ./rtl8192cu-fixes
Build and install the rtl8192cu driver:
sudo dkms install 8192cu/1.10
Refresh modules list:
sudo depmod -a
Blacklist the native kernel driver:
sudo cp ./rtl8192cu-fixes/blacklist-native-rtl8192.conf /etc/modprobe.d/
Disable power management:
sudo cp ./rtl8192cu-fixes/8192cu-disable-power-management.conf /etc/modprobe.d/
Restart your system to use the new driver.
That's it!
For removal, you can use the following commands.
sudo rm /etc/modprobe.d/blacklist-native-rtl8192.conf
sudo rm /etc/modprobe.d/8192cu-disable-power-management.conf
sudo dkms remove 8192cu/1.10 --all