Please note that pi-hole is not Pi specific and you may do better on their ‘discourse’ server.
The entry is due to the router itself not using pi-hole for DNS not devices connected to it.
Unless the router needs to perform automatic updates (EE routers for example in the UK) then it is very very unlikely a home router would need to use pi-hole as it does not need to look up locations (syslog servers etc. for error logs are possible but mine are normally internal addresses).
If you are running VOIP on the router then it should be accessing pi-hole...
Devices connected to the network should use pi-hole as both the DNS and DHCP server in version five and you should turn both functions off on your router. Without that, the IP history becomes a mess on some home routers as they are often not ‘deterministic’ in issuing IPs out i.e. they can hand out random IPs to the same device on each reconnection (do not mix this up with static IPs - they are permanently reserved where deterministic DHCP servers will reuse old IPs for new devices if needed).