Play on 1.9 when the server is on 1.10.2

There is now Plugin for that, because you can't play with a lower Version on a higher Version (some Functions and Blocks are not included at this). But you can go with 1.10/1.10.2 on a 1.9 Server or with the Version 1.10 or 1.9 on an 1.8 Server. All that is possible with the Plugin ViaVersion which is included here at Aternos.

Quote from Brini

There is now Plugin for that, because you can't play with a lower Version on a higher Version (some Functions and Blocks are not included at this). But you can go with 1.10/1.10.2 on a 1.9 Server or with the Version 1.10 or 1.9 on an 1.8 Server. All that is possible with the Plugin ViaVersion which is included here at Aternos.

Sorry I must correct me. The developer of ViaVersion made another Plugin with the functions that you need. It's called ViaBackwards and with this Plugin you can join with 1.9 on 1.10 Servers.

You can try ProtocolSupport, but it only works in one way.