Set unbound cache-max-negative-ttl to lower value
Documentation for cache-max-negative-ttl:
Time to live maximum for negative responses, these have a SOA in the authority section that is limited in time. Default is 3600. This applies to nxdomain and nodata answers.
When internet connection does not work, unbound may receive nodata for dns queries and cache them for 1 hour. Which means that after internet connection is refreshed again, some domain names are not available for about one hour.
Therefore I would propose following change to not cache these negative responses for a long time.
server: cache-max-negative-ttl: 15