Syncing issue with Lightening/CalDav Synology

[BullaMozilla]

Hi,

I have Thunderbird 52.5.2 (64-bit) installed.

In Synology:
1. Installed Calendar
2. Create calendar on my Synology NAS
3. Add test event on Synology calendar

In Lightening:
4. In Right click > new calendar
5. Select 'On the network'
6. Select 'CalDav
7. Enter CalDav link from Synology calendar in 'Location'
8. Click off
9. Enter Calendar name
10. Finish

I am then prompted for my username and password for which I enter the credentials for my NAS.

The calendar does not appear to sync, the test event created in step #3 does not appear in Lightening. Every time I manually sync the calendar I am prompted to re-confirm credentials, which appear to be accepted, but no calendar not updated.

I have added this calendar to my Android mobile device without any problems.

What should my next step be in troubleshooting this?