So many places to look.... First step would be to kill power to devices, router, and modem. Power up modem 1st, wait a good 5 minutes for everything to come back online. Once the lights are back to a normal state on the modem go ahead and power up the router. Wait 5 minutes. Once modem and router are both at their normal operating state, turn on PC or whatever device your using to connect to the router, hopefully your speed issues are gone. This power cycle isn't just something the cable company says to get you off the phone, but a necessary function as it clears any memory and conflicts of IP addressing that may exist in the modem/router. Also, is your modem a modem or a modem/router? If your "modem" has wifi, or more than one place to plug an Ethernet cord into, then it's not just a modem and you should strongly consider enabling bridge mode on the "modem."

Hope this helps.
Also, your devices have to be 802.11ac capable to get the benefit of 802.11ac. Otherwise they will be running on whatever band they can, 802.11a/b/g/n...

It does, you also have the ability to set what you consider to be exceptable voltage fluctuations. Anything outside your set tolerance will sound an alarm.