Nighthawk AC1900/R7000 Slow WiFi

Has a power off for 1 minute then back ON with the ISP modem and router been performed since last update?
Be sure to restart your network in this sequence:
Turn off and unplug modem.
Turn off router and computers.
Plug in and turn on modem. Wait 2 minutes for it to connect.
Turn on the router and wait 2 minutes for it to connect.
Turn on computers and rest of network.

Has a factory reset and setup from scratch been performed? A complete pull of the power adapters for a period of time after the factory reset then walk thru the setup wizard and setup from scratch with a wired PC and web browser. https://kb.netgear.com/22697/How-do-I-install-my-NETGEAR-router-using-the-router-web-interface
Recommend setting the default DHCP IP address pool range to the following after applying and a factory reset: 192.168.#.100 to 192.168.#.200.
https://kb.netgear.com/24089/How-do-I-specify-the-pool-of-IP-addresses-assigned-by-my-Nighthawk-router
https://kb.netgear.com/25722/How-do-I-reserve-an-IP-address-on-my-NETGEAR-router
I would power OFF the ISP modem for 1 minute. Factory reset the router and power it off. Power ON the ISP modem and let it sync. Then power ON the router and walk thru the setup wizard again using a wired PC and a web browser.
Press the back reset button for 15 seconds then release. 
https://kb.netgear.com/9665/How-do-I-perform-a-factory-reset-on-my-NETGEAR-router
https://kb.netgear.com/24233/How-do-I-erase-the-configuration-settings-on-my-Nighthawk-router

Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended.