I found the thread about NBG6817. NBG6816 has very similar hardware, but there is not information about it. Thank you for your help.
https://wikidevi.com/wiki/ZyXEL_NBG6816_(Armor_Z1) has all the information you need, unsupported 5 GHz wlan (quantenna). The rest should be very similar to the nbg6817, but who wants to buy a 200 EUR/ USD device with non functional 5 GHz wlan.
I have zyxel armor z1 (nbg6816) it has the same quantenna chip 5ghz wifi, can someone build a custom build for it if possible, thanks.
You will have to get basic nbg6816 support working for OpenWrt first, right now the device isn't supported at all.
The nbg6817 support should provide some guidance (especially in regards to the firmware format and dual-firmware setup), but there are quite significant differences due to the older SOC and the quantenna integration as well. This porting work can only be done by someone with the device on their desk (e.g. you).
I want to do ti my self but I don't know where to start and how.