iPhone will not automatically connect to one particular wifi network
Let me start this somewhat lengthy soft answer with a suggestion:

See if your landlord will allow you to extend his network by allowing you to add an Airport Express in your apartment. This may be the miracle solution you need to what is otherwise likely to be complex and quite possibly unsolvable problem otherwise.

I strongly suspect you're looking for a technical explanation to your problem. That answer is likely to be lengthy and beyond my expertise. It will also be largely theoretical because without some specific tests performed in your environment, the truth is unlikely to be known, only speculated upon.

The practical answer is that sometimes certain access points don't like certain wifi transceivers. Several years ago I had massive wifi problems with my the just-purchased MacBook Pro and a LinkSys wifi router that had been serving the house successfully for years—and still worked fine for all the other computers in the house, including my iMac G5 running the same version of the OS. 

I applied numerous patches and tried various*tricks* on the Mac. I installed  alternative firmwares onto the Linksys. All with no success.

We installed an Airport Extreme and all of my wifi woes went away. Instantly.

If you're looking for explanation, you'll have to look further. If you want a solution to the problem, see if you can install an AEX, enjoy full use of your device, and don't sweat the details.