Thanks for reaching out here in the Community!
If you are getting an error code or message, refer to this link for common Roku error codes and their recommended actions: How to connect your Roku device to the internet using Wi-Fi or Ethernet
Furthermore, please be advised that Roku Express+ (3931RW) is only compatible with wireless b/g/n networks on the 2.4 GHz spectrum. Ensure that your router is broadcasting a 2.4GHz Wi-Fi network for the device to connect to.
You can also try configuring your router and use wireless channels 1, 6, or 11 which are the preferred channels in North America. Each time you change the wireless channel, you will likely need to apply/save changes before you are prompted to restart the router.
Let us know if there's any difference after.
All the best,
Roku Forum Moderator