Some considerations to improve your situation.

First, please think in terms of bandwidth demands per device. That is, video and gaming takes a lot of bandwidth (BW), voice (like Vonage, Skype, vchat) a bit less, browsing and email demands much less and messaging demands are irrelevant.

Then ask if these demands are going on simultaneously. If so, which demands and from what sources are these demands occurring?

The best thing to do for the high BW and low latency demands is to use hardwired connections to these locations; best examples being the boxes, play station, steaming DVDs, TVs, roku's, AppleTV, PCs used for streAming, etc).. Reason is you get 10x more performance and no noticeable contention over the hardwired connections in comparison to the wireless capabilities of your 1 wireless router. It also leaves only those wireless-only devices (phones and tablets) to use the wireless spectrum of your router.

Next, realize your router has 2 bands. 2.4ghz and the 5ghz bands. Divide up which devices will use which bands so they share the BW capacity of BOTH BANDS in a reasonable manner. If you need more wireless capacity, then get two wireless routers and divide the demand loads across the 4 available wireless bands of both routers - hey, you might actually get to use your DLink and an ASUS to support your 16 devices.

The name of the game here is to manage the available wireless capacity. If you don't you'll not arrive at a happy place. It's the 10 pounds into a 5 pound bag problem you have, not a product technology problem.

Additional configuration options can me asserted to provide BW allocation per device in the QOS options of your router but in the end, you'll need the wireless BW capacity to be available for your devices. So spectrum budget manage is the key.

Hope you follow the engineering aspects of why and what needs to be done.
RegArds..m

Absolutely!

I have many customer homes running on it over several years

Optics has nothing to do with your question

Each TV will require a certain amount of bandwidth for each video stream you wish to view simultaneously. If each TV is accessing the video content wirelessly (very not recommended - better each TV be on a WIRED CONNECTION to the main router), then you must assume there will be some degradation of the HD video when they concurrently operate.

If you step down the video resolution o something less than HD on some tv's you'd get away with using the wireless connectivity.

If you load TomatoUSB Firmware into the AC240 AND adjust the transmit power to 400mW, again you stand a better chance to support 4 concurrent TV's on the router.

As a throughput budget, I've had a pair of wireless 2400, transferring data wirelessly at 1 Gbps using a file transfer program.

Getting back to the optics service provider. If you're paying for the LEAST performing data plan, then it is unlikely the service provide connection will support the 4 concurrent TV's streaming!
..m

Yes.