Your DIR router is not designed for resolving host names of local machines to their IP addresses. At most it just deploys (a) DNS server address(es) to clients via DHCP, that they use to resolve
EXTERNAL names (always "fully qualified domain names (FQDN)" of the form "host.domain-suffix"). These addresses identify either external DNS servers (which your DIR router learns e.g. dynamically from your ISP during Internet connection setup) or it is the address of your DIR router itself, when "
DNS Relay" is enabled, where your DIR router takes all DNS requests for Internet name resolution and relays them to the external DNS servers.
Resolution of local names must be provided by local solutions of your own, for example by operating a local DNS server or by using HOST files.
See similar threads, for example
this one or
that one. Especially
this contribution provides a
link to a solution using a
Raspberry-Pi operating as a local DNS server (by using
dnsmasq).
