Connected to wi-fi but no internet access.

Hi, I'm new here. First of all excuse my poor english.

I have perfect wi-fi connection in my home, everything and everyone can connect to it, but in my work place, we have problems connecting to wi-fi, it says that the device, cell-phone or laptop are connected to wi-fi, but no internet access. I just have 4 months in my work so at first like during a month I was not able to access to internet, but then suddendly one day I had access to internet, that lasted like 2 weeks, and then one day I hadn't and the next day I had again. But lately I can't access to internet, nor the cell-phone nor my computer, but my co-worker sometimes can access perfectly to internet.
Last week I tried everything possible to be able to access to internet but it was not possible, I was 11 of 12 hours not able to access internet, and then all of a sudden, my cell-phone was connected to wi-fi and able to access internet.

My cell-phone is a samsung galaxy s4, is android and it always shows me that message like an infinite loop that says authenticating IP after I put the wi-fi password, but It nevers get to be connected, just tell me that it wasn't able to connect because the conexion was to slow, even if the connection has all its reception.
In my laptop Dell shows me that is connected to the wi-fi but it has no internet access.

Unfortunately I have no access to the modem physicaly, so I can not reset it or disconnect it, but I read it can be done by the computer.

I have search for answers on google and videos or tutorials on youtube and forums, I have tried changing the IP, those numbers in the internet protocol IPv4 changing the IP adress and all those numbers of 8.8.8.8 and 8.8.4.4 and those with the 192 and all of that, I have tried turning off my wirefall, restarting my computer, turning off and on the automatic IP adress, I have tried the ip /config, the command prompt, I tried this:

From the Start Menu, click All Programs, click Accessories, right click the Command Prompt and select 'Run as Administrator'. Into the prompt type the following commands, hitting enter after each command. Include spaces in the commands as shown.

ipconfig /flushdns
nbtstat -R
nbtstat -RR
netsh int ip reset c:\resetlog.txt
netsh winsock reset

Type exit and hit enter. Restart the PC and see if you can now connect.

But none of this worked for me.

And for the cell-phone I've tried the basics like, turning off and on the wi-fi, restarting my cell-phone, and changing the IP adress, I tried this too:

Press these menu options on your phone: Settings > Wi-fi> (Choose your network by tapping on it for a few seconds) > Modify network config

Check "Show advanced options"

Choose IP Settings: "Static"

Enter these details
IP - 192.168.1.253 or 195.168.1.253

Gateway - 195.168.1.1 (I chose 192.168.1.1 since that's my router address)

NPL - 24

Netmask - 255.255.255.0 (I could not find this option)

DNS 1 - 8.8.4.4 or 8.8.8.8 (I had the latter by default and let it stay)

DNS 2 - 4.4.4.4

And save.

at first I thought it worked because my cell-phone showed me a message saying that was already connected to the wi-fi connection but I wasn't receiving any notifications from my social networks like facebook, whatsapp and when I tried to access to internet I couldn't, nothing happened.
Somehow my coworker get to be connected to wi-fi in her cell-phone more often than I, and I don't know what could be the reason.

I have no clue what could be the problem, what am I doing wrong, or what it would be the answer. Also my laptop has windows 7.

I have hope you could help me to fix this problem. Thank you so much for your help and your answers.