I've found a problem with v15 on the VMG8924 using the 802.11a/n/ac (5GHz) band Access Point. It may apply to earlier versions too but this is the first time I've actively used the VMG8924 with 802.11a.
Although my laptops and tablets could detect and connect to the AP (AP reporting channel 100 in use) my HTC One M8 with Cyanogenmod could not *after* I had used the Android OS's Wifi > Advanced > 'Wi-Fi region code' option to set it to "Europe" instead of the default "USA".
It took some digging but I eventually figured out thanks to the table on the Wikipedia List WLAN channels page [ 0 ]. Channel 100 uses the same lower frequency base as channels 102 and 106. The difference is the bandwidth of the channels: 20, 40, or 80 MHz respectively.
Channels 102 and 106 are not available in Europe but the Zyxel is operating in USA mode so when I forced the Android phone to use the European domain it could no longer 'see' the AP.
It seems that although the VMG8924 reports it is using channel 100 it is actually using 102 or 106. The default bandwidth is set to 80MHz which would imply channel 106 (the setting is revealed by pressing the "more..." link next to the "Channel" option).
The only settings I can find that can affect this are on the Network Settings > Wireless > (5G AP) > Other page where it has the option "Regulatory Mode" (Disabled, 802.11h, 802.11d).
It seems to me the default should be set to 802.11h on the 802.11a (5GHz) band which would have it apply automatic DFS (Dynamic Frequency Selection) and TPS (Transmit Power Control).
I'm not sure why there's an option for 802.11d as an alternative since you'd really want 802.11h and 802.11d (which transmits the AP country code in the beacons) working together.
It was set to disabled which appears to be the default, which assumes all channels are legal and would presumably be the USA regulatory domain.
The crucial point is there seems to be no way to tell the AP which regulatory domain it should be operating in which, in the absence of 802.11h being in use, could cause interference.
I'm wondering how this 802.11a 5GHz device gained approvals or is even legal to operate in Europe since it is by default able to use channels and bandwidths that are prohibited due to the possibility of interfering with existing services.
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https://en.wikipedia.org/wiki/List_of_WLAN_channels#5.C2.A0GHz_.28802.11a.2Fh.2Fj.2Fn.2Fac.29.5B17.5D