1. Download Redis caching provider.
    https://github.com/davidjrh/dnn.rediscachingprovider
  2. Install Redis caching provider in Host / Extensions.
  3. Config Redis caching provider in Host / Redis Caching:
    Enable Redis Caching Provider: ON
    Enable Redis Output Caching Provider: ON
    Connection String: my-redis-server:6379
    Compress Cache Items: OFF
    Silent Mode: ON
    Key Prefix: my-web-farm
    cofig-redis-caching-provider

Configuration is stored in web.config.

    ...
    <connectionStrings>
    ...
      <add name="RedisCachingProvider" connectionString="my-redis-server:6379" providerName="DotNetNuke.Providers.RedisCachingProvider" />
    </connectionStrings>
    ...
    <caching defaultProvider="RedisCachingProvider">
      <providers>
    ...
        <add name="RedisCachingProvider" type="DotNetNuke.Providers.RedisCachingProvider.RedisCachingProvider, DotNetNuke.Providers.RedisCachingProvider" providerPath="~\Providers\CachingProviders\RedisCachingProvider\" useCompression="false" silentMode="true" keyPrefix="my-web-farm" />
      </providers>
    </caching>
    ...