Are you sure your 2.4gz is broadcasting/enabled? Does it have a seperate name from your 5gz band? (2 distinct network connections)? Can you upload a screenshot of your 2.4gz settings from your router?
There are plenty of posts here with all the recommended settings for the the Express and setting up the 2.4gz network.
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Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941