Real VNC Server license

Dear all, we need to renew the license for Real VNC Server 5.3 and as the license or this version of VNC_Server is not available, we donwloaded the license for version V6.X.

When we tried to use this license we have this message:

How can we license this version of VNC_Server 5.3? Because the license we get from version 6.X is not compatible any more.