EX6150v2 Netgear AC1200 Range Extender no internet access

I used this WiFi extender for a few years without any issues. Last week, my ISP replaced my fiber optic modem/router and an upgraded model (AdTran 6304W) to increase our data speeds. Now our Netgear EX6150v2 extender isn't working. I have tried the quick setup multiple times using the WPS button method, manually set up on the Netgear installation assistant (mywifiext) method, connecting with an ethernet cable direct to the ISP modem/router...all with the same results. I have tried reseting to factory settings 5 times between trying all these methods. I've called my ISP to make sure everything is working properly on their end (I don't have access to change setting on the ISP AdTran modem/router.)  THE PROBLEM: no matter how I set it up, the result is the same every single time....the extender connects to the AdTran modem, has excellent signal, but NO INTERNET. All our devices in the home can see the extensions and connect to them, but NO INTERNET ACCESS. I've tried my desktop PC, Android phones, tablets, Fire Stick, smart TV, etc. I know it has nothing to do with my PC's internet options settings (no Proxy being used) and I know my AdTran modem/router has the Netgear MAC address listed as allowed and is issuing IP addresses to devices. I can see that the Netgear is issuing my devices IP addresses as well. I have tried the manual workaround for updating the Netgear firmware (the extender can't access the internet to check for updates automatically) by connecting the AdTran directly, downloading and unzipping the latest firware, using the browse option, selecting the new firmware and uploading...but every time it tells me the firmware is incorrect. Does anyone have a REAL solution for this?