I'd seriously looking into the feature Quality of Service and what frequency you devices are connected based on the position of the router and where the device is being used. Since it's a AC router, which is more than enough even on the higher end plans. Sounds like you are exceeding bandwidth with all your devices that are on or you are having interference. Your devices connected basically in a straight line to the wireless router, so you kinda have to figure out how many obstructions are in the way. 5 ghz can go through walls, but not as well as 2.4 ghz. 5 ghz is shorter distance, and 2.4 ghz is longer. This is kinda why if you have one router you want 2 networks seperately so you can easily tell what network the device is on.
Its kinda a complicated to really understand what is using what. But Quality of Service especially if the router remembers the IP restriction on the device, would do a lot of favors, but then you still need to know where the router is positioned and if the antennas are positioned right for the coverage. Dense populated buildings make it more complicated to figure out how to do things.
Just remember if you have a 50 megabit speed plan, and one device is taking 25 mbps, then you only have 25 available. And sometimes another device will create a struggle on what device needs it more.
Distance also decreases wireless signals. You can have a strong signal, but the bandwidth over distance decreases. So ya there is a lot of factors.