External Slave server together with Master he.net

[SinnerLike]

Good day!
I am using dns.he.net for my domain.
I really like your service. Convenient API to update DDNS.

But our Russian Roskomnadzor, in an attempt to censor the Internet and block various VPNs, often "breaks the Internet". Often and for a long time "good" services and sites become unavailable.

I would like to use your service to also use slave DNS somewhere in Russia. But none of the he.net servers are giving away the full zone. How can I customize this?

[snarked]

Set HE servers as secondary.  Primary is hosted elsewhere, and also allows transfers to your other secondaries.