Playing on a Realm in both Java and Win10
Solution 1:
You can't play across Windows 10 Edition and Java Edition.
The reason behind that is that they have a totally different protocol they send packets on. They could have made it with a little effort, so it translates the protocol, but they did not.
If you are playing not on a Realm, but on a Java Edition Spigot server, there have been packet translators already made, like Dragon Proxy, and ProtocolSupportPE, but they might not work in the current version, or might be still Work In Progress, not to talk about that they are not flawless.