Sometimes VPN connections will drop due to inactivity for a period of time specified in the Idle Timeout setting in VPN profiles. This article shows how to configure the Idle Timeout settings so maintain a constant VPN connection without dropping.
Click here for the application note or go to the URL:
https://faq.draytek.com.au/2019/08/30/explanations-of-lan-to-lan-vpn-timeout-values/