How to set up redirection of a request from an internal DNS server to an external one?

Question

linux

There is a rented server with ubuntu and configured VPN. Connection with Mi A1 (LineageOS 15.1 - Android 8.1) goes well, everything works. But I want the connection to occur automatically and when connected to any network. To do this, you need to specify DNS, I entered Google's 8.8.8.8, but the Internet fell off. As I understand it, the internal DNS server does not find anything, but for some reason does not transfer the request to the external DNS provider. How to make it transmit? Or is that not the problem?