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When you switched your internet service, did you also have to switch to a different modem?
By chance do you know the companies that you were using for the internet?
What company did you have before, and what company do you have now?
Changing your ISP should not have done ANYTHING to your internal WiFi. I mean essentially all that was changed was your Public IP address.
So i honestly think it wasnt the change of ISP's, but rather when the modem was swapped out......somethign got unplugged, or reset to factory.
Best thing i can tell you, is to start fresh.
Its VERY difficult to locate the issue when multiple componenst are being used.
So, what i would do...
Factory reset ALL devices down stream from your modem.
Then go through each device and set the correct settings as you woudl like.
Not sure what you mean by DETECT?
How are you detecting? Wifi, or Ethernet cable?
Are you refering to the base station cant detect the express station?
The base station (the Extreme) should have "Create a wireless network" from the Wireless Mode pop-up menu, and then select "Allow this network to be extended" check box.
this is going to take WAY too long.....and since you are using a cell phone as the controller for the express airports.....theres no way i can help you now.
I will opt out for other experts to try.
Best of luck.