USG20W-VPN: Problems establishing Wifi connection / spontaneous reboots

I'm experiencing frequent problems with establishing Wifi connections to my Zyxel USG20W-VPN. Frequently when I switch on the device it's not connecting to the Wifi automatically (like I'd expect). When I'm browsing for the SSID I see it in the list of available Wifi's, but as soon as I try connecting manually I get an error saying the password I entered is wrong. This definitely can't be the case, as the device has been connecting earlier without any problems. It also doesn't work if I enter the password again. So what I started doing was cancelling the password request on my device. When I repeat selecting the SSID I got questioned for the password again. Also rebooting the device wanting to connect to the USG20W-VPN mostly doesn't help / there are some rare cases where it does. Now to the interesting part: If I leave the device on for about 10-15 minutes, then it automatically connects to the Wifi. It doesn't matter whether I just switch it on and leave it the way it is or if I try connecting manually - aborting the password request combined with rebooting 10-15 minutes after the initial try my device connects automatically and then stays connected without any problem.
I also tried rebooting the USG20W-VPN which didn't help on the long run. Maybe it worked a little bit better for a couple of hours/days, but then the problem started again.

I'm experiencing the problem no matter if already a device is successfully connected to the USG20W-VPN or not. I'm experiencing this problem with devices from different manufacturers (Apple as well as Sony) as well as different types of devices (Smartphone, Tablet, Laptop).

My Wifi was initially configured as 2.4GHz and I recently changed it to 5GHz. The problem persists. Since I'm using it on 5GHz I also experienced spontaneous reboots of the USG20W-VPN when using Wifi. As the Log is cleaned after the reboot, I have no chance seeing what caused the reboot.

I initially bought my Zyxel USG20W-VPN with Firmware 4.30(ABAR.0) installed. As soon as the Update was out I updated to 4.31(ABAR.0). The described problem persisted in both version. I changed the frequency to 5GHz about 2 months after I installed version 4.31 (ABAR.0).

I remember having seen the following threads:
https://businessforum.zyxel.com/discussion/965/usg20w-vpn-slow-wireless-connection-establishment#latest
https://businessforum.zyxel.com/discussion/1207/usg20w-vpn-firmware-upgrade#latest
I remember that back in the day when I still was using 4.30 (ABAR.0) I sometimes saw the message about "AP disconnect" in my logs. This is what led me to update to 4.31 (ABAR.0). I can't remember having seen this message since, but the problem still exists. I don't know if the issues described in the mentioned threads should be solved with 4.31 (ABAR.0) or not.

With this behavior the wifi functionality of this device is completely unusable for me. And I really hope to get a fix for my problem.