I have an ASUS DSL-AC56U router (192.168.1.1) and a small LAN, all clients set to DHCP. The router can act as a VPN client (and server if needs be). However, when I try and set up my paid-for VPN (ProtonVPN) from the provider as per their instructions, the internet just cuts out every time I enable it.
I have worked out that the only way I can get it to work if is I go into the router's WAN setting page, and disable 'Connect to DNS server automatically', and then input some public servers (e.g. Quad9).
The exact same thing happens for my other VPN provider.. but their instructions specifically mentioned it?
Is this an expected correct thing? Does it compromise the security of DNS requests made when using the VPN? I'd assumed that the VPN companies had their own DNS servers and securely routed lookups through them?