Deco XE75 not connecting to modem, can't reset
My XE75 was humming along fine for over a year and then the other day woke up with no internet and a solid red light. I think something changed with the ISP modem in the background so I now have a new modem but Deco still won't connect. I tried restting the main Deco with the button on bottom and it's flashing blue. I had seen elsewhere that at this point I should remove the device from the network via the app and set it up again, but now the app won't connect to it.
What do I do now, and am I following the right path to solve the original problem?