Ok I saw your problem…
I think you have not edit correctly the CNAME…
I see your domain name, so I can test the CNAME here https://mxtoolbox.com/ and it did not found it.
In this link, I could verify CNAME for example for my instance cryptpad.linux07.fr. I Choose CNAME Lookup in the orange button and put sandbox.cryptpad.linux07.fr and I see the entry.
Can you verify on your registar zone DNS if the entry is correct ? You will see it with
yunohost domain dns suggest pad.yourdomain.tld
I think for you it must be like
sandbox.pad.maindomain.tld 3600 IN CNAME pad.maindomain.tld.
! Don’t forget the . at the end of the domain