How to add digitalocean nameservers to premiumdns in namecheap

I bought PremiumDns from namecheap 10 hours ago. I added 3 name servers of DigitalOcean to the customDNS but after I enabled the premiumDns, I can’t see the nameservers again. how to fix that, how to add the name servers to the PremiumDns?

The error is DNS error. The site is working now after I added digitaloceans nameservers to custom dns in namecheap. 

The problem There is no custom dns option in the premiumdns of namecheap, and they told me to find another way to add the nameservers.

I’ve contacted namecheap, they told me to contact digitalocean to find an alternative to the custom dns, but they don’t reply.