See if using the included power adapter and plugging directly into wall power helps. Additionally, try disabling the Power Savings feature (if applicable on your Roku device model). This setting will be found under Settings/System/Power/Auto Power Savings/20-min [Disable]
To rule out a network issue, will need to verify if using the 2.4ghz or 5ghz band. If using the 2.4ghz band, try changing the wireless channel (channels 1-11 with 1, 6, and 11 preferred), and if using the 5ghz band, verify that you are not using DFS channels (52-140) or using DFS capability in your router. Roku devices cannot see or use DFS channels, so try assigning manual wireless channel of 36-48 or 149-161.
Feel free to post an update or if you need more help. Include Roku model number from Settings/System/About and include router model number and ISP (internet service provider).
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
Simplest thing to do try is to reset network settings... Your system should reboot but if it doesn't then restart it and try to connect to your WiFi. I've had the same issue as you but only with the stick and after resetting, not simply checking or signing in to the existing WiFi, but reset network settings then hopefully good to go.