I have another issue with the SDIO module. I am using it only with 2.4ghz. If I use a device and browse with it it is incredibly slow. Web sites need up to 10 seconds to load. Other routers with 2.4ghz aren’t that slow.
If I check the RX / Tx Rates in luci These are really low (1mbit) despite reasonable signal strength (-67dbm) and noise (-78 dbm).
You are saying that you are trying to use only with 2.4 GHz frequency, but what are you describing seems to me like interference in the radio caused by other Wi-Fi networks, which is these days common in many areas.
Did you try to use a different channel on 2.4 GHz frequency? I think you should rather check it with some Wi-Fi analyzer like inSSIDer, which will do its job.
In the end, I am curious if you are willing to use an SDIO Wi-Fi card on 5 GHz to see if it is the same or not.
I changed around to put 5ghz on SDIO and 2.4ghz on PCI Express.
Situation is the same. So you might be on to something with interference. I will do a small site survey to see if it is e problem.