Unfortunately, not much you can do with that router other than change the wireless channel. For Roku devices on the 5ghz band, you can select/use any channel 36-48, and 149-161. You will likely need to use the "My Spectrum app" to make these changes, as Spectrum disabled the ability for users to access the router directly via a web portal on these routers.
Additionally, Spectrum has reportedly disabled the 2.4ghz on a fair number of their users' routers without their knowledge. With the SAC2V1K, it is configured as a basic mesh system. If your device disconnects from the 5ghz (normally from weak signal or interference), it is supposed to automatically find and connect to the 2.4ghz band. Since you are having trouble establishing a reconnection, it seems at first glance that the 2.4ghz has either been disabled or is configured improperly.
So.... a couple of things. Call Spectrum and verify that your 2.4ghz is Enabled. You need this as a fallback network for disconnected devices from the 5ghz band. (Alternatively, if the Spectrum app allows you to disable the 5ghz band, you can try that. With the 5ghz band disabled, devices will connect to the 2.4ghz band if it is enabled.)
In order to try and get a connection back to your Roku device, try changing the wireless channel on the router. If you are able to in the settings, try lowering the "Bandwidth or Channel Width" to 40mhz and see if that helps yield more wireless channels and more connection stability. From your RokuTV, perform a Network Connection Reset again (after you change wireless channel). The Network Connection Reset is under Settings/System/Advanced System Settings/Network Connection Reset/Reset Connection.
If still having issues after trying a few different wireless channels, try changing the SSID (network) password from within the router via the Spectrum app. Just use a simpler password (or no password for troubleshooting) and see if connection is made. If using no password (open network) allows you to establish connection easily; and using a password causes you issues, then would begin to consider that something may be wrong with the router or its firmware, or the RokuTV's NIC (wifi module) regarding authentication/connection between the Roku and the network.
At this stage, I would factory reset the router. However, since you lease this router, I would ask for a replacement. Basically, need to graduate the troubleshooting from seeing if a simple setting is causing the issue, or if it is a hardware issue.
Feel free to post back after doing some of the above and report back if anything worked, helped, made worse, better, etc, or if you need more help.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941