Zyxel Router VMG8823-B50B - Settings changes are not applied from WAN access

I've a Zyxel router VMG8823-B50B (Infostrada branded, Firmware 'V5.13(ABIU.0)b5_20190308'). I've configured it for remote (WAN) access (through HTTPS only), and the access works fine: I can reach the login page, login and navigating into the router status and settings. The browser doesn't recognize the certificate as valid for https, but mozilla let me browse with "accepting the risk"

However, every time I change some settings (whatever setting you can think of), the router keep logging me out and the changes are not applied. The logging out is also performed every time I simply refresh the web page (F5).

When logging in from LAN network, it works nice and smooth.

Any idea?