I just reported this bug under Minecraft Pocket Edition as MCPE-25461. I am not sure whether it is a Realms bug, a MCPE bug, or maybe one or more bugs in each.

My subscription was done via Apple's app store. My children are using iPod Touch 6 devices with the most up-to-date iOS, 10.3.3. I am in South Korea and need the Realm to be able to play with them while they are in the United States.

For the 30-day trial period, the Realm worked wonderfully. My children and I built an amazing world that we are still exploring and improving, using survival mode of all things!

Then, I paid a little over $8 USD to start my subscription, and my credit card was charged successfully and everything. However, shortly after that, my children were no longer able to join the Realm and then, the subscription information for the Realm started saying that it did not know which app store I had used.

Before the problem (but after the trial period ended) and afterward, the subscription screen looked like the attached picture. The subscription screen reverted to this mode after Apple refunded my first month's subscription and cancelled it completely.

At this point in time, the Realm is open, but only to me. I have invited two of my children and they have accepted, and one is not able to accept my invitation due to this problem.