I tried to login with an existing account and also with a newly created account.
In both cases Outlook for iOS is not able to synchronize in both ways.
With the normal tools from iOS it works properly using Exchange server connection.
From the logs it has a loop on the settings request from the Outlook app:

ct 21 13:09:37 sogod [7738]: <0x0x557f1ea77520[SOGoActiveSyncDispatcher]> EAS - request for device XXXX: <?xml version="1.0"?>
<!DOCTYPE ActiveSync PUBLIC "-//MICROSOFT//DTD ActiveSync//EN" "http://www.microsoft.com/&quot;>
<Provision xmlns="Provision:">
<DeviceInformation xmlns="Settings:">
<Set>
<UserAgent>Outlook-iOS-Android/1.0</UserAgent>
<Model>Outlook for iOS and Android</Model>
<OS>OutlookBasicAuth</OS>
</Set>
</DeviceInformation>
<Policies>
<Policy>
<PolicyType>MS-EAS-Provisioning-WBXML</PolicyType>
</Policy>
</Policies>
</Provision>

Oct 21 13:09:37 sogod [7738]: <0x0x557f1ea77520[SOGoActiveSyncDispatcher]> EAS - response for device XXX: <?xml version="1.0"?>
<!DOCTYPE ActiveSync PUBLIC "-//MICROSOFT//DTD ActiveSync//EN" "http://www.microsoft.com/&quot;>
<Provision xmlns="Provision:">
<AllowHTMLEmail>1</AllowHTMLEmail>
</Provision>