Here is some things you can do.
1: Unplug the power cables to your Roku and Router.
2: After 10 to 30 seconds, plug them back in.
3: Go into network on your Roku.
(NOTE: IF YOU ARE ON ANY OTHER NETWORK, THE STEPS WILL STILL WORK.)
4: Go to 192.168.0.1. (Most Wi-Fi controller's use this IP address.)
5: Log in.
6: Go to Wireless Setup and then Advanced SSID Setup.
7. Select a SSID.
8: Name the network whatever you want.
9. Put the network UNDER WPA2 - WPA3.
10: Click apply changes.