Q: Airport Express IP issue

I have two Airport Express's in my house.

The first one is an older Airport Express (The one that plugs directly into the wall socket) which I use as a wireless to Ethernet bridge for my Satellite decoder - occasionally I use it on my xbox 360.

The other one is a newer Airport Express (which looks like a white Apple TV) which I also use as a wireless to ethernet bridge for my iMac which is far away from the router. I can use the built-in wireless of the iMac but because of its location I get around 30-35MBPS and with the Airport Express I get around 85-90MBPS.

Now, here is my issue: The older airport express works fine and never has any issues but its no longer being found with Airport Utility. I've tried Airport utility on my Macbook Pro, iMac and iPhone. None of them find this Airport yet it remains connected and giving internet to my satellite fine.

The more annoying issue is with the newer Airport Express - That shows up fine in Airport Utility but for some reason after some time of non-use like almost always when the iMac goes to sleep and then I return to using it the Internet does NOT work and the IP address on the computer shows up as INVALID like 169.xxx.xxx. I can fix it by resetting the airport express using airport utility - just restarting the airport is enough.

The problem is not the iMac or its ethernet port. I know because I also tried and had the exact same issue using two Windows PC's.