The idea that bandwidth is “divided” among devices is simply flat out wrong. Any technician that says this is either grossly misinformed or lying through their teeth. Bandwidth is only divided when devices are pulling down significant bandwidth simultaneously. Devices that are idle do not impact bandwidth.
According to tests I’ve read about, the AC66U should be able to handle over 800 Mbps of throughput from the Internet to your network. The fact that your speeds are cut in half make me think that your router may be malfunctioning. Or perhaps the tests are not representative of real-world results.
Make sure your firmware is up to date on your router. If your firmware is current and you are still having the problem, unfortunately the only way to prove that your router is the problem is to test another high performance router in its place (either the same or different model, as it just might be your particular unit is the problem). Good luck and let me know if you are able to test another router and if you can get your full bandwidth.