drinkingbird
Part of the Furniture
Ok, manually switching off primary wan won't switch to secondary wan
Try unplugging the cable (both for fail over and fail back). If it works reliably, get yourself a couple smart plugs. You can even set up a timed routine to automatically switch over at a certain time and switch back later.
But if it were me I'd be fighting and escalating with the ISP. They are clearly saturating their bandwidth. If it is 600/600 I'm assuming it is fiber which means they can just groom you over to a different fiber on your street and hopefully alleviate it. Unless it is somewhere further upstream in which case they have even bigger problems they need to solve, and I'm sure you're not the only one complaining.