This error indicates that your A record for mydomain.io cannot be resolved to the issuing server. So you’re right in thinking this is a DNS issue.
You mention that you pointed your DNS to to Digital Ocean’s DNS. If this change was made after adding your A record then you will need to add the A records in Digital Ocean’s Domain manager as that’s where your new records will be held, not with your original registrar.
Instead of pinging your domain I would check with a tool like this: https://mxtoolbox.com/DNSLookup.aspx
From there you can verify that there is a valid A record pointing to the correct IP. You can also use the tool to check for duplicate entries or incorrect configuration such as using a CNAME record in addition to an A record, and verify the name servers you are using.
NB If you have IPV6 entries on your DNS configuration these will take precedence over IPV4. Sometimes registrars can add these by default. IPV6 addresses are indicated with AAAA as opposed to the IPV4 version of A
Finally to check propagation globally you can use this tool: https://dnschecker.org/