Why might name resolving not work on mobile devices?

Question

Domain Name System

It took to open several sites on the local network by domain name.
Set up Synology dns server.
On the PC, everything works correctly and nslookup gives the necessary ip addresses.
But it does not work on mobile devices in the same wifi network, it gives an ERR_NAME_NOT_RESOLVED error, although the dns server is pinged.
Tried on iphone 5 se and several android devices. Dns server changed in phone settings.
In an attempt to catch the problem, I specified non-existent dns servers in the Wi-Fi settings, cleared the dns cache in browsers, turned off proxies, vpn, and traffic savers in browsers, but I still managed to go to sites that I had not visited before. How is this possible?