Netbox DNS plugin - TTL for nameserver records

I'm running Netbox community edition 4.6.0 with the Netbox DNS plugin, and have got it mostly working as I need. One thing that is confusing me however, is that I can't see how to get the TTL set for the nameserver records for a zone?

There doesn't appear to be any way to edit the values, which are showing as blank at present. I've got the default TTL set for each zone, which I would have thought should be inherited by the ns records, but it's not.

Probably missing something obvious here, but any tips appreciated!