This is the risk of using shared spectrum devices.
While your internet provider is 'mainly concerned' with the 5Ghz network, does that mean he turned OFF 2.4ghz? It would be unlikely that they would completely disable 2.4ghz, as that would stop legacy devices from working, as well as dramatically reduce range in many environments, resulting in the need for far more access points.
So, since the problem started right after that change was implemented, I'm guessing that change has something to do with your sudden problems.
This is the exact reason I never plan to put 2.4ghz mics into my inventory. Since it's a shared spectrum, anyone nearby can do something that will potentially muck with the signal. Sharing spectrum with a TV station is less risky, as new TV stations typically don't pop online mid show...