Can't connect to web despite full-strength connection on wifi

Question

Alright, I've scowered the internet and read several other topics, but none of the suggestions presented there helped fix my problem, so I'm throwing this at the support discussions. Thanks in advance for your help/suggestions/advice!
My wife and I live in an apartment building that provides wifi for the entire building. It's password protected, and about every 24 hours, you'll get a splash page (I don't know if that's the proper term) where you have to scroll through the building's terms & conditions and click "I Agree" before you can continue surfing. We had absolutely no problems with this setup for 9 months, then suddenly we lost all connection in our unit about 10 days ago. Between my wife and I, we have a Macbook Pro, iMac, Windows 7 laptop, 2 iPhones, iPod touch, and iPad mini, and none of these devices could connect. Unfortunately, the router and modem for our building's connection is located in another renter's unit, so I can't just reset them whenever I want. However, we did have them try this several times, but it didn't help. I even asked them to rotate the antennas on the wifi router, to see if somehow they got bumped and our unit became a dead-spot. Unfortunately this didn't work either. I even purchased a brand-new iPad mini and it never connected, right out of the box.
If I wander the halls of our building, I am able to get online in Safari and browse the internet on both the Macbook Pro and the iOS devices. However, once I go back into my apartment, I get the message that I am no longer connected to the internet, despite having the full signal at the top of my screen. Then, for no apparent reason two days ago, our Windows 7 laptop regained connection in our unit, and has consistantly worked since then. Just today, I renewed the DHCP lease on the iMac, and it has been connected for several hours. Occasionally, the Macbook Pro will connect in our apartment until, but the connection literally lasts for about 60 seconds before it goes back into the "You're not connected to the internet" mode. I've tried renewing the DHCP lease on the Macbook Pro as well as the iOS devices, but it doesn't seem to do anything.
Here's what my wifi connection page on the Macbook Pro looked like yesterday when I was in my apartment and I didn't have internet connection, despite the fact it said I was connected:
and then here's the MacBook Pro's wifi connection page looks like when I am out in the hallway and able to connect to pages on Safari:
Lastly, (I don't know if this is helpful or not) but here's the connection page on the iMac, when it is working...taken at the same time as when the Macbook Pro was connected when I was out in the hallway:
A couple of thoughts:
Does the iMac have a larger wifi range than the other devices? It seems that in the past, I've been able to see/connect to more wifi networks on my iMac than I have on my Macbook or iOS devices. Could it just be that the Windows laptop and the iMac are able to reach our network, while the iOS devices are having a hard time for some reason? Particularly odd that all devices worked for 9 months then suddenly stopped.
I'm asking the apartment manager if they she contact our local cable company about replacing the modem and/or wireless router. Given that I've never seen these (they're in someone else's unit) I have no idea how old they are, or if they need firmware updates.
Do you think this is a problem with the wifi network (range/dead spots), the router/modem hardware, or a problem with my computer/devices?
I'd love to get all my devices connected again (seriously, they all worked for 9 months before stopping suddenly), but I'm kind of working on the MacBook Pro first, and then will try and get the other things sorted out.
Thanks for your help!
-J- (writing from the hallway of my apartment complex!)

Answer 1

Take each of the following steps that you haven't already tried, until the problem is resolved.
Step 1
Ask whoever is in charge of the router to restart it. Many problems are solved that way.
Step 2
Back up all data.
Launch the Keychain Access application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Keychain Access in the icon grid.
Enter the name of your wireless network in the search box. You should have one or more "AirPort network password" items with that name. Make a note of the name and password, then delete all the items. Quit Keychain Access. Turn Wi-Fi off and then back on. Reconnect to the network.
Step 3
The administrator may need to change other settings on the router. See the guidelines linked below:
Recommended settings for Wi-Fi routers and access points
Step 4
Make a note of all your settings for Wi-Fi in the Network preference pane, then delete the connection from the connection list and recreate it with the same settings. You do this by clicking the plus-sign icon below the connection list, and selecting Wi-Fi as the interface in the sheet that opens. Select Join other network from the Network Name menu, then select your network. Enter the password when prompted and save it in the keychain.