For those who are seeing the problem where Google sites load and other sites don't when you're associated to the Guest Network on the Extender base station, I have an idea for a brief experiment to figure out what the underlying problem is, so Apple will have a more concrete bug description to work with.
This experiment is only for those who see Google sites load, but most other sites do not load. If you have a problem where no sites load at all, this experiment is not for you.
On your Mac that you're using as the Guest Network Extension test client, as a temporary test, try adjusting your MTU down from 1500 to something much lower like 1420. You can do this in "System Preferences > Network > Wi-Fi > Advanced > Hardware", by changing "Configure:" from Automatic to Manually, and by changing MTU from "Standard (1500)" to Custom, and typing in 1420. Then hit "OK" to dismiss the sheet and then "Apply" to make the MTU change take effect.
See if that lets you connect to sites besides Google. Unlike most websites, Google automatically limits their TCP MSS (Maximum Segment Size; kind of like a TCP-layer equivalent to the IP-layer concept of an MTU) to 1380, which would be equivalent to an IP-layer MTU of 1420. This might be why you can get to Google but not other sites, which use the much more common default MSS of 1460.
If that lets you surf the web fine from the guest network of the extender base station, then there may be an issue where something's not controlling the MTU or the MSS correctly. So your client might be trying to send full-sized 1500-byte MTU frames that, unbeknownst to it, are too big for one of the network links between it and "the Internet". These frames might be getting dropped or corrupted in a way that keep the client machine from being able to discover that it needs to use a smaller MTU.
If 1420 works, then try 1492. If 1492 also works, try 1496.
If MTU's up to and including 1492 work, but 1493+ don't work, then you probably have a PPPoE link involved in your broadband connection, and something's going wrong with MTU or MSS control involving the combination of the PPPoE link and Guest Network Extension.
If MTUs up to and including 1496 work but 1497-1500 don't, then something may be going wrong involving MTUs for VLAN-tagged frames (Apple uses VLAN tagging to separate Guest Network Extension traffice from your main LAN traffic on the connection between the extender and the main AP).
This test could probably be done in less than 15 minutes:
- Reproduce the problem without changing anything. Join the Guest Network of the extender base station (your AirPort Express) and confirm that google.com loads but other websites don't.
- Set the 1420 MTU on the Wi-Fi interface of your Mac test client. Try to load a few webpages including Google.com and others. If the other pages still don't load, you're done. This problem probably isn't related to MTU's or MSS's.
- If 1420 worked, then the problem is related to MTU's or MSS's. Set your Mac to a 1492 MTU. Try to load a few webpages including Google and others. If the other pages don't load anymore, you're done. This problem is MTU related, but might not be specific to PPPoE or VLAN tagging (or your ISP requires an even smaller PPPoE MTU than usual).
- If 1492 worked, try 1496. Try to load a few webpages including Google and others. If the other pages don't load anymore, this means the problem is related PPPoE MTU's. If they still work, then it means the problem is likely related to VLAN tagging's effects on MTU/MSS. Either way you're done. I suppose you could try 1497 just for giggles, but if 1496 works and 1500 doesn't, 1497 will most likely fail just like 1500, and it still points to MTU problems related to VLAN tagging.
