It's not a Synology issue, but alas is with the Orbi.
I've got a call with a Netgear Engineer tomorrow afternoon.
I've been through a whole host of testing, gone back to the ISP router to prove that has no issues, downgraded the Orbi firmware (which it auto-upgraded). Latest thing was to re-load an old config from back in April (which I know was good).
So far 6 hours without a DNS issue. I'm not convinced it's fixed it, at the issue had occurred at various intervals from 2 to 112 hours.
However, I did go through the settings to see what was different in this configuration and the only difference was NTP was set to use the Netgear NTP server instead of uk.pool.ntp.org.
In the raft of reading up on the issue I've done one guy mentioned it could be NTP related. So I switched the NTP server to uk.pool.ntp.org, had second thoughts and went back to the Netgear server. When I looked to confirm that change reversal I got an error message: "* Warning: The Orbi Router failed to connect to NTP server, please check your internet connectivity or try another NTP server".
I left it saved at using the Netgear NTP server, but I'm wondering if it is an NTP issue and the Netgear NTP server is broken is that what is stopping the problem from occurring?
Just a thought..... So can you check what your NTP settings are? Maybe set the NTP server to Netgear, see if you get the error I do? And more importantly if it fixes the problem for you?