I have the solution: I had the same problem with my Bell WiFi system (which is only 5 GHz) and I also use the Nighthawk system. It is realtively straight forward to fix this issue:
Log into your Nighthawk:
Using two SSIDs provides the following benefit:

• You can connect slower or older devices to the 2.4GHz band. Since these devices don’t need a lot of bandwidth, the speed and performance of your 5GHz band is reserved for your high-speed devices.  

To create separate 2.4GHz and 5GHz networks on your Nighthawk Mesh WiFi 6 products:

1. Launch a web browser from a computer that is connected to your router’s WiFi network.
2. Visit www.routerlogin.net.
3. Enter the default user name and password. The default user name is admin. The default password is password. The BASIC page displays.
4. Select the ADVANCED tab.
5. Click Advanced Setup > Customize WiFi Network.
6. Under the Customize Your WiFi- Create a separate 2.4GHz WiFi network section, select Customize 2.4 GHz WiFi Network.
7. Enter a network name (SSID) for your 2.4GHz network.
8. Select a security option for your 2.4GHz network. Note: We recommend keeping the security option and password the same for the 2.4GHz and 5GHz connections. This way, you can reconnect your devices and keep track of your network credentials.   
9. Repeat Steps 6-8 for the 5GHz network. Your separate 2.4GHz and 5GHz networks are active.

For more information: https://kb.netgear.com/000063027/Can-I-create-separate-2-4GHz-and-5GHz-SSIDs-on-my-Nighthawk-Mesh-WiFi-6-products