If it were me, I would notify the apartment management in writing using registered mail with return receipt of the possible security issues you have found. I would indicate in the letter that if...

Both bands should be stable. I think you are on the right track by changing the channels as I would suspect some type of interference. Try different combinations of channels, modes (such as 11bgn...

Get a router (and modem) that exceeds your needs and does all you want. You will grow into it. Sometimes it pays to spend more and other times it is a waste. Modems are much like cars... Different...

On your device, try deleting (forget) the 5 Ghz network and then reestablishing it. Also remember that the 5Ghz does not have the WIFI distance that 2.4Ghz has however it is generaly faster. When you...

When you switched ports, did the same ports work or did the same devices work? If the same devices work, then try new ethernet cables for the failing devices. If it is the same ports on the router...

Many devices only operate on the 2.4 Ghz and not the 5Ghz. You need to check each device's specs.