When you create a Minecraft Realm you are given the option to enable coordinates but the option is not visible in the settings after that. You can however enable coordinates via commands. Only the owner and operators can enable coordinates and cheats do not need to be enabled so your achievements are still obtainable.
Step 1: Open the Chat
Step 2: Type /gamerule showcoordinates true
Step 3: Click ‘Enter’
Here’s my video where I show a complete walkthrough of enabling coordinates and pvp on a Minecraft Realm.