Zyxel VMG8825-T50 creating hidden network after factory reset.

So after factory reset i noticed that on my PC showed up Hidden network. I checked it on wifi analyzing app and it's only on 5g. It definitely comes from router (after turning 5g off it disappears). I didn't switch option to hide SSID and my 5g network SSID is visible. Any ideas why does it do that? I tried several factory restart but it sill appears.
 It seems it's slowing down 5g network - before restart it was 300mbps now it's 120 top.

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