This application note covers a scenario where the WAN MTU setting can affect connectivity and data flow over a VPN tunnel.

https://faq.draytek.com.au/2019/10/16/client-cannot-open-web-interface-if-remote-dial-up-vpn-pptp-is-used/

Other related articles are:

Unable to Access Remote Router Web Page over IPsec VPN Tunnel

https://faq.draytek.com.au/2015/08/14/unable-to-access-remote-router-web-page-over-ipsec-vpn-tunnel/

Lower VPN MSS to access the peer router’s Web UI through VPN

https://www.draytek.com/support/knowledge-base/4678