Check your firmware version: Settings/System/About - if its 9.3.0, there is a known bug that prevents/degrades screen mirroring connections when the Roku is connected via 2.4Ghz - firmware version 9.4.0-4155 mostly fixes this issue.
Check for a new firmware version manually: Settings/System/System Update/Check now - if it offers 9.4.0-4155, take the upgrade.
(Note: there are known issues with 9.4.0 on some models, especially Express+ models, so you'll have to decided whether the upgrade is worthwhile to you if available...)
StreamerUser Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.