Why are some Internet pages not being opened when using the M4 Mini?

The following problem was described for a customer: Some Internet pages cannot be accessed when using the Mine Media M4, while others can. The problem here is not to be found in the M4 Mini – as a malfunction – but in the DNS settings. Specifically, we were able to reconstruct the following case:
In the settings of the Mine Media M4 Mini Router, under Network Setting-LAN Setting, the DNS server was deposited by Google. The router has tried to resolve websites via the Google DNS server. This worked for all devices in the network as long as the router was ONLY operated with SIM cards. On the other hand, there were problems as soon as the “customer LAN” was also used by the organizer. Then suddenly not all websites could be accessed.
It turned out that the DNS server is blocked in the customer’s network, which disrupted the function of the M4 Mini.
The solution: In the menu under Network Setting-LAN Setting the DNS1 entry turned off. The router was then restarted and all websites could be accessed again.
Furthermore, the following was also observed with regard to Internet access and Mine Media: Sometimes it is necessary to make the DNS settings individually for the connected device (notebook) and to enter a DNS server there, e.g.