Hey there! We are keeping an eye on this and will let you know when the fix is in!
Mojang Status
 @MojangStatus
8 Dec 2020
We are aware that some players are having issues joining Realms right now. We’ll let you know when this is resolved!
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If you'd like to submit a suggestion, please visit feedback.minecraft.net/hc/en…. ^CD

Dec 31, 2020 · 3:31 PM UTC