First, the Express has a single band WiFi radio, so if your router is only broadcasting on 5 GHz, you need to enable the 2.4 GHz radio. The next thing is the channel number. The 2.4 GHz band has channels 1-14, but the top three are not permitted in North America so Roku devices will not see them. The best channels to use on 2.4 GHz is 1, 6 or 11. These are the only three channels that don't have overlap with each other. Most routers are set to auto-select the channel, so you may need to set it manually.
Dan
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee, just another user.