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Xbox players run on bedrock edition.
Java edition players and bedrock edition players can't play together
Sorry, I guess I didn't read your post right. I thought one of the players was playing java edition.
Tablets use also bedrock edition. So technically xbox and tablet players should be able to play together.
But it might be that xbox players are limited to the official servers and that they are not able to join 3rd party servers through an IP address. This is a limitation from the creators on Mojang implemented on some consoles.
I can't check if xbox also has this limitation as I don't have one.
