Historically speaking – the Wi-Fi industry is about 25-years old… a mere toddler in the grand scheme of things. But we have now matured to a place where we should be strong enough to accept the use of Checklists!
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I think 40MHz channel is supported in 2.4 GHz band as well.
Experts can confirm…
You’re correct @shamshad. I believe the recommendation being made is to use only 20 MHz channels in 2.4 GHz, not that they are not supported. This way congestion is reduced.