If you have a cable ISP (Xfinity/Comcast, Cox, or AT&T), you may have to verify that your 2.4ghz band is configured to use the b/g/n protocol.
Your Express can only see and use the 2.4ghz band so this is first setting to look at, as well as verifying that your 2.4ghz band is actually on/enabled and broadcasting. Increasingly, these cable ISP routers are being locked down (managed by ISP, not user-editable/configurable) so may have to contact them and let them know you need your "2.4ghz Wireless Mode to be set to b/g/n".
Here are instructions for various cable ISP in case you have a model that is still configurable. If the above is not applicable to you, post back with more details of your issue, including Roku model number (Settings/System/About), your router model numer, and your ISP.
You can try connecting to a mobile hotspot if available to you on the 2.4ghz band to verify that your Express units are capable of making a connection. This should help you verify that a network (router) setting is your culprit.
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