Gateway on Ubuntu server 16.04. Problems accessing DNS servers?

Question

Domain Name System

Good day!
The question is this: there is a local network of 60 computers (most Windows 7, some Windows XP), no Active Directory. All computers in the LAN are assigned a static address, not by DHCP.
Internet distribution is done through a software router on Ubuntu Server 16.04 (a simple computer with two network cards), the provider provides a dynamic ip-address via pppoe.
Also in the network there are 5 managed d-link switches, but working as simple unmanaged ones, and several simple unmanaged d-links.
Actually, the problem is that when the provider issues an ip-address from its range of gray addresses, for example: 10.21.9.7, then problems with access to DNS servers begin, access to web pages starts to fail, the traceroute command shows that the packets are not delivered.
However, when the provider issues ip from the list of white addresses, everything is in order.
I called the provider, they said that everything is in order with them and we have a problem. I think so too. I'm waiting for advice from more experienced comrades, where to dig?

Answer 1

when the provider gives out an ip-address from its range of gray addresses, for example: 10.21.9.7, then problems with access to DNS servers begin

There can be many reasons, but it is not difficult to check.
First you need to see which DNS servers the provider issued, whether they are available (banal ping) from the router, whether they are available from the client. Whether other DNS servers are specified on the client.
In general, check, then it will be possible to draw conclusions, and not guess.

All computers in the LAN are assigned a static address, not by DHCP.

Not the best solution, both in terms of stability and handling.
I recommend distributing all addresses on the local network via DHCP - both static and dynamic.

Answer 2

Buy a permanent IP.