When on Minecraft mobile IOS. I play in a realm with my friend. However, if Minecraft has been opened and not fully closed, realms aren't found. You have to fully close Minecraft in the multitasking screen. So if you go off Minecraft for even a second and you come back you can't get onto the realm. You have to fully close it and reopen it. My friend has the exact same issue so it is not just my iPad. 

Basically you can't get onto realms without fully closing and relaunching Minecraft.