The only thing you need to do to connect a realtek modem in linux is to install the right driver for your device.

First download the latest driver for your device because the driver supplied by default with your modem may be outdated:

run this code in terminal to get the model number:

lsusb

now you will see a line like:

Bus 001 Device 004: ID 0bda:8176 Realtek Semiconductor Corp. RTL8188CUS 802.11n WLAN Adapter

here RTL8188CUS is the model number, get yours and remember it or note it.

Now go to this site and download the right driver for your device. search for your model no. and download it from the table which says "unix"  . note the supported linux kernel range given. for now the supported kernel range from 2.6.18 to 3.9 for RTL8188CUS.

now check if your kernel is supported. if not, then you might have to roll back to some kernel which is supported (a risky task), because these drivers are greatly dependent on linux kernel version. for now i am assuming your kernel is supported.

code for kernel check:

uname -r

Now install the driver:

extract the archive.

run these codes one by one:

sudo chmod -R 777 path_to_the_extracted_folder

hint: path_to_the_extracted_folder=   ~/Downloads/extracted_folder_name    (if the folder is extracted in Downloads)

cd path_to_the_extracted_folder

sudo ./install.sh

Now you will be able to see your network card in the network manager....

kudos..........