OK - I managed to discover I could tcpdump the traffic on the router then look at it with wireshark. I should point out I have very little experience here!
I can see the Garmin Scales ask for DHCP and then receive an IP from the Turris router
Scales then ask Turris for DNS clock.garmin.com
Turris replies, and Scales then NTP successfully with clock.garmin.com
Scales then ask Turris for DNS gold.garmin.com (1st)
Scales then ask Turris for DNS gold.garmin.com (2nd)
Turris responds with DNS details for gold.garmin.com (2nd)
Turris responds with DNS details for gold.garmin.com (2nd)
Scales then ask Turris for DNS gold.garmin.com (again) (1)
Scales then ask Turris for DNS gold.garmin.com (again) (2)
Turris responds with DNS details for gold.garmin.com (again) (2)
Turris responds with DNS details for gold.garmin.com (again) (2)
This DNS request and response seems to go on for the next 50 secs or so, and eventually the Scale concludes it can not connect, so times out. The wireshark log shows the Turris not to respond to the first DNS query, but respond twice to the second query.
On my phone, I have a Garmin app, this communicates over wifi successfully with gold.garmin.com. I have also tried another Garmin device (Edge 1000) over wifi, that works fine too though I note it doesn’t do any NTP (presumably as its a GPS device, so knows the time from there).
Perhaps the problem is with either the format DNS response from the Turris or the interpretation of the DNS response by the Garmin Scales?