Realm instances not seeing each other

I'm working on a migration from 5.6.x to 5.7.1.

We have two servers that we want to sync up.  The secondary instance is able to see the Primary instance, at least in the General settings tab, however the Primary is unable to see the secondary.  We have verified firewall settings and the ports used in the realm initialization settings.  We can also determine two way communication on the port configured.

Is there additional configurations necessary to get the instances to sync with each other outside of the general settings options?