One of Nulls Clash Royale's most secret and best servers has released a free version for everyone. Null's Royale - is one of the most stable servers from developers OpegitStudio. To be honest, the server from OpegitStudio has been around for a long time, but it was paid.
Nulls Royale is probably the only available private server on which you can participate in battle 2 by 2. This was Gl-Clash Royale, but this option had removed so far and it's not clear if they will add anything at all. At the moment I'm doing a lot of work on the list of all private servers and a small table will be prepared soon. In which you will see all the advantages of each server.